Scan mirrors for laser radar

ABSTRACT

Laser radar systems include a pentaprism configured to scan a measurement beam with respect to a target surface. A focusing optical assembly includes a corner cube that is used to adjust measurement beam focus. Target distance is estimated based on heterodyne frequencies between a return beam and a local oscillator beam. The local oscillator beam is configured to propagate to and from the focusing optical assembly before mixing with the return beam. In some examples, heterodyne frequencies are calibrated with respect to target distance using a Fabry-Perot interferometer having mirrors fixed to a lithium aluminosilicate glass-ceramic tube.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application61/728,999, filed Nov. 21, 2012, and U.S. Provisional Application61/753,786, filed Jan. 17, 2013; and is related to U.S. PatentApplications “LOW DRIFT REFERENCE FOR LASER RADAR,” “LASER RADAR WITHREMOTE LOCAL OSCILLATOR,” and “RADAR SYSTEMS WITH DUAL FIBER COUPLEDLASERS,” which are filed concurrently with the present application, allof which are incorporated herein by reference.

FIELD

The disclosure pertains to laser radar systems.

BACKGROUND

Laser radar systems provide simple, convenient, non-contact measurementsthat aid in single-operator object inspection. Laser radar systems areparticularly useful for applications in which large objects, such asaircraft, automobile, wind turbine or satellite parts, are to bemeasured. Some conventional laser radar systems are described in U.S.Pat. Nos. 4,733,609; 4,824,251; 4,830,486; 4,969,736; 5,114,226;7,139,446; 7,925,134; U.S. Patent Application Publication 2011/0205523,and Japanese Patent 2,664,399 which are incorporated herein byreference. In such laser radar systems, a laser beam is directed to andscanned over a target surface, and portions of the laser beam that arereflected or scattered back to the laser radar are detected andprocessed to provide target information. Useful beam scan rates can belimited by encoder speeds, and distance calibration tends to driftduring use. In addition, costs of such systems are great. Accordingly,improved laser radar methods and apparatus are needed.

SUMMARY

In some examples, distance measurement systems comprise a scanningreflector that includes at least two reflective surfaces. A firstrotational stage is coupled to the scanning reflector and configured toscan an optical measurement beam with respect to a target and receive anoptical return beam from the target in response to the scanned opticalmeasurement beam. In some embodiments, a light source is coupled todirect the optical measurement beam to the scanning reflector and anoptical receiver is situated to receive the return optical beam from thescanning reflector. A signal processor is configured to provide anestimated target distance for at least one target location based on thereceived return optical beam. In other examples, the light source iscoupled to direct the optical measurement beam to the scanning reflectoralong an axis that is orthogonal to an axis of propagation of themeasurement beam from the scanning reflector to the target. In typicalexamples, the two reflective surfaces are situated at an angle of 45degrees or 135 degrees with respect to each other. In furtherembodiments, an optical fiber is configured to receive the measurementbeam from the light source and direct to the measurement beam to thescanning reflector from a fiber end surface, and to receive the returnbeam at the end surface.

In representative examples, distance measurement systems include acorner-cube situated to receive the measurement beam from the endsurface of the optical fiber. A beam focusing optic is configured toreceive the measurement beam from the corner cube and focus themeasurement beam at a target surface, wherein the corner cube ismoveable with respect to the beam focusing optic so as to produce ameasurement beam focus at a selected target distance. In some examples,a second rotational stage is configured to change a propagationdirection of the measurement beam by the rotation of the firstrotational stage. In a typical example, the first rotational stage issituated to produce a selected beam elevational angle and the secondrotational stage is configured to produce a selected beam azimuthalangle. In some alternatives, the second rotational stage is configuredto rotate the end surface of the optical fiber. In some embodiments, thetwo reflective surfaces are defined on respective plane reflectors or atsurfaces of a solid prism such as a pentaprism.

In some disclosed examples, the light source is configured to produce asecondary beam based on the measurement beam such that the opticalreceiver is situated to receive the secondary beam, and wherein thesignal processor is configured to provide the estimated target distancefor at least one target location based on the received return opticalbeam and the secondary beam. In an embodiment, the estimated targetdistance for the at least one target location is based on a differencefrequency associated with the received return optical beam and thesecondary beam. In representative examples, the light source isconfigured so that the optical measurement beam and the secondaryoptical beam are frequency chirped optical beams, and the estimatedtarget distance for the at least one target location is based on adifference frequency associated with the received return optical beamand the secondary beam and a chirp rate. In representative examples, thesignal processor is coupled to the light source and configured to selecta frequency chirp of the frequency chirped optical measurement beambased on an estimated or measured target distance.

In representative examples, a corner-cube situated to receive theoptical measurement beam and a beam focusing optic is configured toreceive the measurement beam from the corner cube and focus themeasurement beam at a target surface. The corner cube is moveable withrespect to the beam focusing optic so as to produce a measurement beamfocus at a selected target distance. The light source is configured todirect the secondary beam to the corner cube and the optical receiver issituated so as to receive the secondary beam after propagation in thecorner cube. The light source is coupled to an optical fiber systemconfigured to produce the optical measurement beam and the opticalsecondary beam from a common input optical beam, wherein the opticalfiber system is further configured to receive the optical return beamand provide a combined beam that includes the return and secondaryoptical beams to the receiver optical system.

Representative methods include directing a measurement optical beam soas to be reflected by at least two reflective surfaces of a scan mirrorand rotating the scan mirror so as to scan the measurement beam along ascanned axis. An optical beam returned by the scan mirror from thescanned axis is received, and, based on the received returned beam, atleast one target distance is estimated. In some examples, the scanmirror is rotated to direct the measurement beam to a referencereflector so as to produce a reference return beam. Based on thereference return beam, a deviation of the scan axis from an intendedscan axis is determined.

Calibration devices for optical measurement apparatus include a supportstructure, and a first reflective optical surface secured to the supportstructure. The first reflective surface defines a circulating opticalpath having an optical path length based on the support structure, andthe first reflective surface is configured to introduce an optical beamso as to propagate along the circulating optical path. A referencephotodetector is configured to receive at least two optical beams havinga relative delay based on the optical path length and produce an outputsignal, and a signal processor associates an output signalcharacteristic with the optical path length. In some examples, the firstreflective surface is configured to couple a plurality of delayedoptical beams to a photodetector, wherein each of the delayed opticalbeams of the plurality of delayed optical beams has an optical delaycorresponding to an integer multiple of the optical path length of thecirculating optical path. In other examples, a second reflective surfaceis secured to the support structure. The second reflective surface isconfigured to couple a plurality of delayed optical beams to thereference photodetector, wherein each of the delayed optical beams ofthe plurality of delayed optical beams has an optical delaycorresponding to an integer multiple of the optical path length of thecirculating optical path. In representative embodiments, the firstreflective surface and the second reflective surface are secured to thesupport structure so as to define a Fabry-Perot resonator, and theoptical path length corresponds to a separation of the first and secondreflective surfaces. In some examples, the second reflective surface isconfigured to couple a plurality of delayed optical beams to aphotodetector, wherein each of the delayed optical beams of theplurality of delayed optical beams has an optical delay corresponding toan integer multiple of the optical path length of the circulatingoptical path.

In typical examples, the optical path length defined by the supportstructure is based on a portion of the support structure having acoefficient of thermal expansion of less than 0.5·(10⁻⁶)/° C.,0.5·(10⁻⁷)/° C., or 0.2·(10⁻⁷)/° C. According to some examples, theoptical path length defined by the support structure is based on aportion of the support structure comprising a glass ceramic such as alithium aluminum silicon oxide glass ceramic having a coefficient ofthermal expansion of less than 0.1·(10⁻⁶)/° C.

In some embodiments, at least one of the first reflective surface andthe second reflective surface is non-planar. In a typical example, atleast one of the first reflective surface and the second reflectivesurface is non-planar so that the Fabry-Perot resonator is a stableresonator. In other examples, the first reflective surface and thesecond reflective surface are situated along an axis that includescenters of curvature of the first reflective surface and the secondreflective surface so that the introduced optical beam is directed so asto propagate off-axis. In still other alternatives, first, second, andthird reflective surfaces are secured to the support structure so as todefine a ring resonator, and the optical path length is corresponds to apropagation distance based on separations of the first, second, andthird reflective surfaces. In additional examples, a container isconfigured to retain the support structure and the first reflectiveoptical surface so that the circulating optical path is defined withinthe container. A temperature controller is thermally coupled to thecontainer and configured to set a container temperature.

Laser distance measurement apparatus comprise a probe beam sourceconfigured to direct a probe beam to a target. A measurement detector isconfigured to receive at least a portion of the probe beam from thetarget. A reference length defines a circulating optical path having anoptical path length based on an ultralow coefficient of thermalexpansion (ULE) support structure. The reference length is configured toreceive a reference beam and direct the reference beam so as topropagate along the circulating optical path. A reference detector isconfigured to receive the reference beam from the circulating opticalpath. A signal processor is coupled to the measurement detector and thereference detector and configured to establish an estimate of a targetdistance based on a received portion of the probe beam from the targetand a received portion of the reference beam from the reference length.In other embodiments, the reference beam received from the circulatingoptical path is associated with propagation of the reference beam alongtwo or more multiples of the optical path length of the reference lengthand the signal processor is configured to establish the estimate of thetarget distance based on received portions of the reference beamassociated with the two or more multiples of the optical path length.According to representative examples, the reference length includes afirst reflector and a second reflector secured to the support structureso as to define the circulating optical path. In typical examples, thefirst reflector and the second reflector are arranged to define a FabryPerot resonator, and the optical path length is associated with aseparation of the first reflector and the second reflector.

In other embodiments, the support structure is a ULE rod and the firstreflector and the second reflector are situated at opposing ends of theULE rod. According to a representative example, the ULE material is oneor more of a lithium aluminum silicon oxide ceramic or fused quartz. Insome embodiments, a fiber coupler is configured to produce themeasurement beam and the reference beam from a common optical beam, andthe reference length includes an input optical fiber and an outputoptical fiber configured to receive the reference beam from the fibercoupler and direct the reference beam from the circulating optical pathto the reference detector.

In a particular example, the common optical beam is a frequency sweptoptical beam, and the signal processor is configured to establish theestimate of the target distance based on a frequency difference betweenthe received portion of the probe beam from the target and a localoscillator beam and at least one frequency difference associated withreceived portions of the reference beam associated with two or moremultiples of the optical path length of the reference length. Typically,a hermetically sealed container is configured to retain the referencelength, and a temperature controller is coupled to the hermeticallysealed container to select a temperature associated with the referencelength. In some cases, the reference length is defined in an opticalfiber or other refractive medium. In other embodiments, the referencelength includes a plurality of reflective surfaces arranged to define aring resonator, and the optical path length is associated withseparations of the plurality of reflective surfaces. In furtherexamples, the reference length includes at least one reflective surfacesituated so that propagation of the reference beam along the opticalpath length is associated with two reflections at different locations onthe at least one reflective surface.

Methods comprise directing a reference optical beam to an opticalresonator that defines an optical path length so as to produce areference beam portion associated with a transit along the optical path.The optical path length is based on a dimension of an ultralow thermalexpansion (ULE) support. The reference beam portion is received at oneor more photodetectors, and a propagation length associated with thereference beam portions is estimated based on the received portion. Insome examples, a plurality of reference beam portions is produced andthe reference beam portions are associated with corresponding transitsalong the optical path. In typical examples, the reference optical beamis a swept frequency optical beam, and the propagation length estimatesare based on frequency differences between the reference beam portions.According to other examples, the frequency differences are obtained bydirecting the plurality of reference beam portions to a photodetectorand obtaining heterodyne frequencies associated with interference of thereference beam portions. In some examples, the ULE support is a lithiumaluminum silicon oxide ceramic. In other alternatives, a correspondenceof a frequency sweep associated with the swept frequency optical beamand target distances is established. In still further examples, a probeoptical beam is directed to a target, the probe optical beam being aswept frequency optical beam having a frequency sweep corresponding tothe frequency sweep of the reference optical beam. A differencefrequency between the probe optical beam as received from the target anda local oscillator optical beam is obtained. Based on the differencefrequency associated with the probe optical beam and the correspondenceof the frequency sweep and target distances, at least one targetdistance is estimated.

Measurement apparatus comprise a measurement beam source coupled toprovide a probe beam and a reference beam. An optical system includes afocus adjustment optical system having at least one movable opticalelement so as to focus the probe beam at a target surface. At least onephotodetector is configured to receive a portion of the probe beamreturned by the target surface to the focus adjustment optical systemand a portion of the reference beam from the focus adjustment system. Insome examples, the reference beam propagates as a collimated beam in thefocus adjustment system. In some examples, the focus adjustment opticalassembly is situated so that the portion of probe beam returned by thefocus adjustment system and the reference beam have a common number oftraverses of the movable optical element of the focus adjustment opticalassembly as received at the at least one photodetector. Inrepresentative examples, the movable optical element is a movableretroreflector such as a corner cube. In other alternatives, the focusadjustment optical system includes a reference beam retroreflector andreference beam reflector, wherein the reference beam retroreflector issituated to receive the reference beam from the movable retroreflectorand direct a displaced reference beam to the reference beam reflectorthrough the retroreflector, and the reference beam reflector isconfigured to direct the displaced reference beam back to the referencebeam retroreflector. According to some examples, the measurement beamsource includes an optical fiber configured to provide the probe beamand the reference beam, and the focus adjustment optical system isconfigured to deliver the portion of the probe beam returned by thetarget surface and the reference beam from the reference beamretroreflector to the optical fiber. In still other examples, themeasurement beam source includes an optical fiber configured to providethe probe beam and the reference beam, and the focus adjustment opticalsystem is configured to deliver the portion of the probe beam returnedby the target surface and the reference beam to the optical fiber. Inother examples, the focus adjustment system includes a return reflectorsituated to receive the probe beam from the retroreflector and directthe probe beam back to the reflector and the optical system includes alens situated to receive the probe beam and focus the probe beam at atarget distance. Typically, the lens is situated to direct the portionof the probe beam returned by the target surface into the optical fiber.In some examples, the measurement beam is a swept frequency beam and thephotodetector is configured to produce a signal at a differencefrequency that is associated with a target distance.

According to some embodiments, the measurement apparatus comprises acompound rotational stage that includes an azimuthal rotational stageand an elevational rotational stage secured to the azimuthal rotationalstage, wherein the optical system is secured to the elevational stage sothat the probe beam is directed to the target surface based on anelevational angle and an azimuthal angle. In typical embodiments, asignal processor is configured to determine a target distance estimatebased on the difference frequency. In representative examples, a beamdivider is configured to provide the probe beam and the reference beamfrom the measurement beam. In some examples, the beam divider is basedon division of wavefront or division of amplitude. In a representativeexample, the beam divider includes a beam splitter situated to receivethe measurement beam and transmit one of the probe beam or the referencebeam, and at least one of the probe beam or the reference beam isdirected by the beam splitter to the focus adjustment optical system. Insome examples the beam splitter is a polarizing beam splitter (PBS), anda wave plate is situated so as the reference beam is coupled from thePBS to the movable optical element in a first state of polarization andfrom the movable optical element to the PBS in a second SOP that isorthogonal to the first SOP. In convenient examples, the first andsecond SOPs are linear SOPs, and the at least one wave plate isconfigured to provide a ¼ wave retardation.

In other alternatives, the beam divider includes at least one opticalsurface configured to select a first portion of a measurement beam crosssection as a probe beam and a second portion of the measurement beamcross section as a reference beam. According to some examples, the atleast one optical surface is a refractive surface having a firstcurvature in a surface area corresponding to the probe beam portion ofthe measurement beam and a second curvature in surface areacorresponding to the reference beam portion of the measurement beam. Inother examples, the at least one optical surface includes a firstreflective surface area situated so as to reflect either the probe beamportion of the measurement beam or the reference beam portion of themeasurement beam. In some embodiments, the first reflective surface areais situated to provide a first beam divergence for the probe beam and asecond beam divergence for the reference beam, wherein the first beamdivergence and the second beam divergence are different. In stillfurther examples, the beam divider includes a first optical surface anda second optical surface configured to select a first portion of ameasurement beam cross section as a probe beam and a second portion ofthe measurement beam cross section as a reference beam. In someembodiments, the first surface includes a reflective area associatedwith either the probe beam portion or the reference beam portion, andconfigured to reflect either the probe beam portion or the referencebeam portion to the second surface. The second surface has a reflectivesurface area situated to reflect the received beam portion from thefirst surface so that the probe beam portion and the reference beamportion propagate along a common axis with different beam divergences.According to representative examples, the second surface includes atransmissive area configured to transmit the measurement beam to thefirst surface and an optical fiber is situated to couple the measurementbeam to the second surface. The optical fiber includes a fiber surfacesituated at the second surface, wherein the fiber surface couples themeasurement beam to the second surface.

Methods comprise selecting a focus of a probe beam at a target with afocusing optical assembly and receiving a portion of the probe beam fromthe target returned to the focusing optical assembly. A reference beamis directed to the focusing optical assembly and returned from theoptical assembly. A target distance is estimated based on the receivedportion of the probe beam and the reference beam returned from thefocusing optical assembly. In some examples, the focus adjustmentoptical assembly is situated so that the portion of probe beam returnedby the focus adjustment system and the reference beam have a commonnumber of traverses of at least a portion of the focus adjustmentoptical assembly. According to representative examples, the focus of theprobe beam is selected with a movable optical element of the focusingassembly, and the portion of the probe beam returned from the target andthe reference beam have a common number of traverses of the movableoptical element of the focus adjustment optical assembly. In someembodiments, the movable optical element is a movable corner cube or aroof prism such as an air corner cube or an air roof prism. In someembodiments, the measurement beam is divided based on a division ofmeasurement beam wavefront so as to form the probe beam and thereference beam. According to some examples, the measurement beam isdivided by directing the measurement beam to an optical surface havingsurface areas with different curvatures or different reflectivities. Insome examples, the measurement beam is divided by directing themeasurement beam to a first surface configured to transmit a first crosssectional area of the measurement beam and reflect a second crosssectional area of the measurement beam portion. In representativeembodiments, the target distance is estimated by mixing the probe beamportion returned from the target and the reference beam at a detector,and determining a frequency difference between the probe beam portionand the reference beam.

Beam dividers configured to provide a probe beam and a reference beamfrom a measurement beam comprise at least one optical surface configuredto select a first portion of a measurement beam cross section as a probebeam and a second portion of the measurement beam cross section as areference beam and produce different beam divergences for the probe beamand the reference beam. In some examples, the at least one opticalsurface is a refractive surface having a first curvature in a surfacearea corresponding to the probe beam portion of the measurement beam anda second curvature in surface area corresponding to the reference beamportion of the measurement beam. In other examples, the at least oneoptical surface includes a first reflective surface area situated so asto reflect either the first portion or the second portion of themeasurement beam. In still other examples, the first reflective surfacearea is situated to provide a first beam divergence for the probe beamand a second beam divergence for the reference beam, wherein the firstbeam divergence and the second beam divergence are different. Accordingto some examples, the first surface includes a reflective areaassociated with either the probe beam portion or the reference beam, andconfigured to reflect either the probe beam or the reference beam to thesecond surface. The second surface has a reflective surface areasituated to reflect the received beam portion from the first surface sothat the probe beam and the reference beam propagate along a common axiswith different beam divergences. In further examples, the second surfaceincludes a transmissive area configured to transmit the measurement beamto the first surface. In some examples, an optical fiber is situated tocouple the measurement beam to the second surface.

Beam pointing systems comprise a first rotational stage configured toprovide a rotation about a first axis and a second rotational stagecoupled to the first rotational stage, and configured to provide arotation about a second axis that is not parallel to the first axis. Arotatable optical element is coupled to the second rotational stage, andan optical system is situated to provide a probe beam to the rotatableoptical element. In some examples, the first axis is an azimuthal axisand the second axis is an elevational axis, or the first axis is anelevational axis and the second axis is an azimuthal axis. Inrepresentative examples, the rotatable optical element is situated toreceive the probe beam from the optical system along a propagation axisparallel to the second axis. According to some examples, the rotatableoptical element has a planar reflective surface situated to receive theprobe beam so that the probe beam is directed to a target location basedon a first rotation angle and a second rotation angle associated withthe first rotational stage and the second rotational stage,respectively. In some examples, the rotatable optical element is apentaprism. According to some embodiments, the rotatable optical elementis situated to receive the probe beam from the optical system along anaxis parallel to the first axis. In other alternatives, the opticalsystem is configured so as to be stationary with respect to rotations ofthe first and second rotational stages. In additional examples, theoptical system includes a photodetector configured to receive a portionof the probe beam returned from a target. In still further examples, acamera is secured so as to be rotatable about the first axis so as toimage a target field of view.

In still other alternatives, rotatable optical element optical issituated to receive the probe beam from the optical system along an axisparallel to the second axis. An optical fiber is coupled to the opticalsystem so as to deliver a measurement beam to the optical system, andthe optical system is configured to produce a probe beam and a referencebeam based on the measurement beam. In some embodiments, the firstrotational stage and the second rotational stage include respectiveencoders, and a signal processor is coupled to the encoders so as todetermine a pointing direction of the probe beam based on encodersignals. According to some examples, the optical system includes atleast one optical element that is translatable to adjust a focusdistance of the probe beam. In particular embodiments, the opticalsystem includes a corner cube and an objective lens, wherein thetranslatable optical element is the corner cube situated so as to vary apropagation distance associated with the objective lens. In someexamples, the optical system is configured to produce the reference beambased on a portion of the measurement beam directed to the corner cube.In some convenient examples, the optical system is configured to couplea portion of the probe beam returned from a target and the referencebeam into the optical fiber. In other examples, a camera is coupled soas to be rotatable about the second axis, and configured to image atleast a portion of a target.

Distance measurement systems comprise a scanning reflector that includesat least two reflective surfaces. An optical system is configured todirect an optical probe beam to the scanning reflector. A firstrotational stage is coupled to the scanning reflector and configured toscan an optical probe beam with respect to a target based on a rotationof the scanning reflector, wherein the optical system is configured toreceive an optical return beam from the target in response to thescanned optical probe beam. According to some embodiments, an opticalreceiver is situated to receive the optical return beam from the opticalsystem, and a signal processor is configured to provide an estimatedtarget distance for at least one target location based on the opticalreturn beam. In still other alternatives, the optical probe beam isdirected to the scanning reflector along an axis that is orthogonal toan axis of propagation of the optical probe beam from the scanningreflector to a target. In some examples, the two reflective surfaces ofthe scanning reflector are situated at an angle of 45 degrees or 135degrees with respect to each other. In yet other embodiments, an opticalfiber is configured to receive the optical probe beam from the lightsource and direct to the optical probe beam to the optical system and toreceive the return beam from the target.

In a particular example, the optical system comprises a corner cubesituated to receive the optical probe beam from the optical fiber. Abeam focusing optic is configured to receive the optical probe beam fromthe corner cube and focus the measurement beam at a target surface,wherein the corner cube is moveable with respect to the beam focusingoptic so as to produce a beam focus at a selected target distance. Inadditional examples, a second rotational stage is configured to change apropagation direction of the optical probe beam to the target, whereinthe first rotational stage is situated to produce a selected beamelevational angle and the second rotational stage is configured toproduce a selected beam azimuthal angle. In some examples, the tworeflective surfaces are defined at surfaces of a solid prism such as apentaprism. In further examples, the light source is configured toproduce a secondary beam, and the optical system is configured to couplethe secondary beam and the return probe beam to a photodetector. Thesignal processor is configured to provide the estimated target distancefor at least one target location based on the received return opticalbeam and the secondary beam. In some examples, the estimated targetdistance for the at least one target location is based on a differencefrequency associated with the received return optical beam and thesecondary beam.

Laser radar systems comprise a measurement beam source configured toprovide a swept frequency optical beam. An optical system is coupled toreceive the swept frequency optical beam and produce a probe beam and areference beam, select a focus distance of the probe beam, and couple aprobe beam portion from a target and the reference beam to a detector. Aprobe beam pointing system includes an elevational rotational stage andan azimuthal rotational stage, wherein the elevational rotational stageis coupled to the azimuthal rotational stage. A rotatable reflectivesurface is coupled to the elevational stage, and configured to receivethe probe beam and direct the probe beam to a selected target location.In some examples, the probe beam is coupled to the rotatable reflectivesurface along an elevational axis of rotation or along an azimuthal axisof rotation.

Optical measurement apparatus comprise a beam pointing system comprisingan elevational stage and an azimuthal stage. An optical system issecured so as to be rotatable with the azimuthal stage. The opticalsystem is configured to receive a measurement beam and shape themeasurement beam for delivery to a target area as a probe beam, whereinthe target area is based on an elevational angle and an azimuthal angleestablished by the beam pointing system. The optical system alsocombines a portion of the probe beam returned from the target with aportion of the measurement beam in an optical fiber, and includes arotatable reflective surface situated so as to establish an elevationalangle for the probe beam. A signal processing system is configured toprovide an estimate of a target distance based on the combined beam. Insome examples, a measurement detector is coupled to the combined probebeam and measurement beam portions, wherein the signal processing systemis electrically coupled to the measurement detector and configured toprovide the estimate of the target distance based on an electricalsignal from the measurement detector. According to other examples, themeasurement detector is secured so as to be rotatable with the azimuthalstage or so as to be fixed with respect to elevational and azimuthalrotations provided by the beam pointing system. In representativeexamples, the rotatable reflective surface is a surface of a planemirror, a surface of a prism such as a pentaprism, a surface of apentamirror, and can be defined by a multilayer dielectric coating. Insome examples, first and second measurement lasers are coupled toprovide a dual wavelength measurement beam to the optical fiber of therotatable optical system.

In additional examples, a reference length includes a Fabry-Perotresonator coupled to receive a portion of the dual wavelengthmeasurement beam. A reference detector is optically coupled to theFabry-Perot resonator and electrically coupled to the signal processingsystem so as to provide a reference electrical signal, wherein thesignal processing system is configured to provide the estimate of targetdistance based the reference electrical signal. In typical examples, thereference electrical signal includes signal portions corresponding to aplurality of transits of a cavity defined by the Fabry-Perot resonator.In other examples, the rotatable optical system includes at least onetranslatable optical element configured to adjust a focus of the probebeam, wherein the portion of the measurement beam combined with thereturned portion of the probe beam is coupled through the at least onetranslatable optical element. In some examples, the portion of themeasurement beam and the returned portion of the probe beam are combinedso as to have corresponding optical paths in the at least onetranslatable optical element or to have a common number of transits ofthe at least one translatable optical element. According to someexamples, the at least one translatable optical element is a corner cubeor a roof prism.

In further examples, an enclosure is configured to retain the referencelength, and a temperature controller is thermally coupled to theenclosure and configured to establish a temperature of the enclosure. Inadditional embodiments, a pointing laser that produces a visible opticalbeam is configured so that the visible optical beam is coupled with thedual wavelength measurement beam to the optical fiber of the rotatableoptical system. In some alternatives, the optical fiber of the rotatableoptical system is a polarization retaining single mode optical fiber. Inyet other examples, a camera is secured so at to be rotatable with theazimuthal stage and situated to image along a propagation axis of theprobe beam.

In other embodiments, measurement apparatus include a dual wavelengthfiber-optic transmitter and receiver system that includes first andsecond lasers coupled to provide a combined beam to an input/outputoptical fiber, a reference length coupled to receive a portion of thecombined beam and to couple a reference beam to a reference fiber, and athermally controlled enclosure configured to retain at least thereference length. A beam shaping optical system is coupled to receivethe combined beam from the input/output optical fiber of thetransmitter/receiver system. Typically the beam shaping optical systemincludes a beam focusing lens and at least one translatable focusadjustment optical element configured to focus a dual wavelength probebeam at a target surface. In some examples, an azimuthal stage isconfigured to direct the dual wavelength probe beam to the targetsurface on a selected azimuthal angle, and the beam shaping opticalsystem is configured to direct the dual wavelength probe beam along anaxis of rotation of the azimuthal stage. According to some alternatives,a rotatable reflective surface is configured to receive the dualwavelength measurement beam and direct the dual wavelength measurementbeam along a selected elevational angle. In some embodiments, a camerais configured to image a target surface along the axis of rotation ofthe azimuthal stage. In further embodiments, a rotatable reflectivesurface is configured to receive the dual wavelength probe beam anddirect the dual wavelength probe beam along a selected elevationalangle. A cold mirror is configured to transmit the dual wavelength probebeam to the rotatable reflective surface and reflect imaging opticalradiation to the camera. In other cases, an elevational stage and anazimuthal stage are configured to select a target location, wherein thebeam shaping optical system is secured to so as to be rotatable based onthe selected elevational angle. In further examples, a signal processoris configured to provide a target distance estimate based on portions ofthe probe beam returned from the target. In additional examples, thebeam shaping optical system is configured to form a combined localoscillator (LO) beam, wherein the signal processor is configured toprovide the target distance estimate based on the portions of the probebeam returned from the target and the combined LO beam. In someexamples, the LO beam is formed so as to have an optical path in thetranslatable focus adjustment optical element corresponding to anoptical path in the translatable focus adjustment optical element of thereturned probe beam portions to and from the target. In some cases, thetarget distance is estimated based on a difference frequency between theprobe beam and the LO beam. In typical examples, a first detector and asecond detector are configured to receive returned probe beam portionsand LO beam portions corresponding to the first laser and the secondlaser, respectively, and the target distance is estimated based ondifference frequencies between the probe beam and the LO beam at thefirst detector and the second detector.

The foregoing and other features, and advantages of the disclosedtechnology will become more apparent from the following detaileddescription, which proceeds with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a portion of a representative laserradar that includes a local oscillator beam and measurement beam thatare directed to a corner cube that provides focus adjustment for themeasurement beam.

FIG. 2 is a perspective view of a portion of a representative laserradar system that includes a remote local oscillator beam.

FIG. 3 is a schematic diagram of a portion of a representative laserradar system that includes a reflective surface situated at or near afiber input and configured as part of an optical path of a localoscillator beam.

FIGS. 4-7 are schematic diagrams illustrating optical systems configuredto produce a local oscillator beam and a measurement beam from a commoninput beam based on a division of input beam wavefront.

FIGS. 8-9 are schematic diagrams illustrating optical systems configuredto produce a local oscillator beam and a measurement beam from a commoninput beam based on a division of the input beam wavefront and toreduced unwanted multiple reflections with an angled fiber beam couplingsurface.

FIG. 10 is a schematic diagram of a representative folded optical systemthat includes an angled fiber beam coupling system and that can producemeasurement and local oscillator beams from a common input beam.

FIGS. 11-12 are schematic diagrams of additional catadioptric opticalsystems that can produce a measurement beam and a local oscillator beam.

FIG. 13 is a schematic diagram of a zero power lens configured toproduce measurement and local oscillator beams from a common input beam.

FIG. 14 is a schematic diagram illustrating a reflective surface that isprovided with portions having two different surface curvatures forseparation of an input beam into measurement and LO beams.

FIG. 15 illustrates a representative all refractive (dioptric) opticalsystem that can produce measurement and local oscillator beams from acommon input beam.

FIGS. 16A-16C illustrate representative refractive (dioptric) opticalelements that can produce measurement and local oscillator beams from acommon input beam.

FIG. 17 illustrates a representative optical system that can producemeasurement and local oscillator beams from a common input beam using apolarizing cube beam splitter (PBS) and quarter wave retarders.

FIGS. 18-19 illustrate a representative optical system that can producemeasurement and local oscillator beams from a common input beam usingpolarizing beam splitters and retarders.

FIG. 20 is a perspective view of a representative laser radar opticalsystem in which measurement and local oscillator beams are obtained froma common input beam based on beam states of polarization.

FIGS. 21A-21B illustrate heterodyne beat frequencies associated withupchirped measurement and LO beams, and down-chirped measurement and LObeams.

FIG. 22 is a schematic diagram of a representative reference lengthstandard using a Fabry-Perot interferometer configuration.

FIG. 23 is a schematic diagram of a representative reference lengthstandard based on multiple reflections within a dimensionally stablecontainer.

FIGS. 24-25 illustrate additional reference length configurations.

FIG. 26 is block diagram of a laser radar that includes a laser chirpcontroller configured to permit automatic or manual change of laserchirp rates.

FIG. 27 is a block diagram of a method of estimating a range to a targetusing variable laser chirp rates.

FIGS. 28 and 29A-29C illustrate laser radar systems with representativescanning assemblies.

FIGS. 30A-30B are elevational and plan views of a laser radar systemthat includes a pentaprism scanner.

FIGS. 30C-30D illustrate detection and correction of wobble inducederrors based on a curved reference mirror in laser radar systems such asthose of FIGS. 30A-30B.

FIG. 31 is a schematic diagram of an optical fiber based beam combiningsystem for a dual laser laser radar system.

FIG. 32 is a schematic diagram of an optical fiber based beam combiningsystem for a dual laser laser radar system that includes opticalcirculators and a pointing laser.

FIG. 33 illustrates a representative laser tracking system that includesa partially transmissive mirror situated to couple an optical beam to adetector for estimation of a beam pointing direction or direction error.

FIG. 34 illustrates a representative laser tracking system that includesa partially transmissive mirror situated to couple an optical beam to adetector for estimation of a beam pointing direction or direction errorand a bulk optical system situated to deliver and receive optical beams.

FIG. 35 illustrates a representative laser radar that includes apartially transmissive reflector configured to direct a measurement beamto a target and produce a local oscillator beam.

FIG. 36 illustrates a laser radar system in which combinedcounterchirped laser beams are coupled to a focusing and scanning systemwith a single mode fiber.

FIG. 37 illustrates a laser radar system in which a single mode opticalfiber couples a chirped laser to a polarization based beam splittingsystem that produces a measurement beam and an LO beam.

FIG. 38 illustrates a laser radar system in which a single mode opticalfiber couples a chirped laser to a polarization based beam splittingsystem that produces a measurement beam and an LO beam. Beam focus isadjustable with translation of a corner cube.

FIGS. 39-40 illustrate laser radar system that includes a rotatablefolding mirror so as to scan a measurement beam over a target.

FIG. 41 is a schematic diagram of a multi-beam laser radar system.

FIG. 42 illustrates a representative range signal processing receiver.

FIG. 43 illustrates a laser radar that includes a folding mirror with anaperture.

FIGS. 44A-44B are schematic diagrams of an optical assembly configuredto produce a dual laser measurement beam and two remote LO beams.

FIG. 45 is a schematic diagram of an optical assembly configured toproduce a dual laser measurement beam and two LO beams.

FIG. 46 illustrates an optical fiber system that produces a dual lasermeasurement beam, and combined a returned beam with each of two localoscillator beams.

FIG. 47 illustrates an optical fiber system that produces a dual lasermeasurement beam, and combined a returned beam with each of two remotelocal oscillator beams.

FIG. 48A is an elevational view of an optical system configured toreceive a dual laser measurement beam and two LO beams and scan themeasurement beam with respect to the target.

FIG. 48B is a view of a portion of the optical system of FIG. 48A.

FIG. 49 illustrates an optical system configured to receive a dual lasermeasurement beam and at least one LO beam from corresponding opticalfibers in which an LO reflector is secured to a scanning assembly.

FIG. 50 is a schematic diagram illustrating a compact optical systemthat produces parallel measurement beams at first and second wavelengthsand non-parallel LO beams in orthogonal polarizations.

FIG. 51A is a schematic diagram of a polarization maintaining basedfiber optical system configured to produce first and second localoscillator beams and a dual wavelength measurement beam.

FIG. 51B illustrates heterodyne frequencies associated with a movingtarget location using the system of FIG. 51A.

FIGS. 52-54 illustrate representative laser radars/laser trackers thatinclude systems such as those disclosed above.

FIG. 55 is a block diagram of a measurement system based on an amplitudemodulated optical beam.

FIG. 56 is a block diagram of a representative method of tracking atooling ball that is secured to a substrate or target.

FIG. 57 is a block diagram of a representative manufacturing system thatincludes a laser radar or other profile measurement system tomanufacture components, and assess whether manufactured parts aredefective or acceptable.

FIG. 58 is a block diagram illustrating a representative manufacturingmethod that includes profile measurement to determine whethermanufactured structures or components are acceptable, and if one or moresuch manufactured structures can be repaired.

DETAILED DESCRIPTION

As used in this application and in the claims, the singular forms “a,”“an,” and “the” include the plural forms unless the context clearlydictates otherwise. Additionally, the term “includes” means “comprises.”Further, the term “coupled” does not exclude the presence ofintermediate elements between the coupled items.

The systems, apparatus, and methods described herein should not beconstrued as limiting in any way. Instead, the present disclosure isdirected toward all novel and non-obvious features and aspects of thevarious disclosed embodiments, alone and in various combinations andsub-combinations with one another. The disclosed systems, methods, andapparatus are not limited to any specific aspect or feature orcombinations thereof, nor do the disclosed systems, methods, andapparatus require that any one or more specific advantages be present orproblems be solved. Any theories of operation are to facilitateexplanation, but the disclosed systems, methods, and apparatus are notlimited to such theories of operation.

Although the operations of some of the disclosed methods are describedin a particular, sequential order for convenient presentation, it shouldbe understood that this manner of description encompasses rearrangement,unless a particular ordering is required by specific language set forthbelow. For example, operations described sequentially may in some casesbe rearranged or performed concurrently. Moreover, for the sake ofsimplicity, the attached figures may not show the various ways in whichthe disclosed systems, methods, and apparatus can be used in conjunctionwith other systems, methods, and apparatus. Additionally, thedescription sometimes uses terms like “produce” and “provide” todescribe the disclosed methods. These terms are high-level abstractionsof the actual operations that are performed. The actual operations thatcorrespond to these terms will vary depending on the particularimplementation and are readily discernible by one of ordinary skill inthe art.

For convenience in the following description, the terms “light” and“optical radiation” refer to propagating electromagnetic radiation in awavelength range of 300 nm to 10 μm, but other wavelengths can be used.Such radiation can be directed to one or more targets to be profiled,detected, or otherwise investigated. This radiation is referred toherein as propagating in one or more “beams” that typically are based onoptical radiation produced by a laser such as a laser diode. As used inthis application, beams need not be collimated, and propagatingradiation in a waveguide is referred to as a beam as well. Beams canhave a spatial extent associated with one or more laser transversemodes, and can be substantially collimated. Wavelengths for whichoptical fibers or other optical waveguides and coherent laser sourcesare readily available are convenient. In some examples, laser diodes atwavelengths around 1550 nm are used.

For convenience, beams are described as propagating along one or moreaxes. Such axes generally are based on one or more line segments so thatan axis can include a number of non-collinear segments as the axis isbent or folded or otherwise responsive to mirrors, prisms, lenses, andother optical elements. The term “lens” is used herein to refer to asingle refractive optical element (a singlet) or a compound lens thatincludes one or more singlets, doublets, or other compound lenses. Insome examples, beams are shaped or directed by refractive opticalelements, but in other examples, reflective optical elements such asmirrors are used, or combinations of refractive and reflective elementsare used. Such optical systems can be referred to as dioptric,catoptric, and catadioptric, respectively. Other types of refractive,reflective, diffractive, holographic and other optical elements can beused as may be convenient. In some examples, beam splitters such as cubebeam splitters are used to separate an input beam into a transmittedbeam and a reflected beam. Either of these beams can be arranged toserve as measurement beam or a local oscillator beam in a coherentdetection systems as may be convenient. Beam splitters can also beprovided as fiber couplers, and polarizing beam splitters are preferredin some embodiments.

In the disclosed examples, laser radar systems are configured to scan aprobe or measurement beam over a scan path that can be a polygon,portions of a closed curve, a raster, a w-pattern, or other pattern, andscanning can be periodic or aperiodic. In response to a measurement beamor a probe beam directed to a target, a return beam is obtained based onreflection, scattering, diffraction, refraction, or other process at thetarget. Evaluation of the return beam permits estimation of targetproperties. The examples below are provided with respect to a laserradar that is configured to, for example, provide an estimate of surfacetopography based on portions of an optical beam directed to a surfacethat are returned to a receiver. The disclosed methods and apparatus canalso be incorporated into laser tracker systems.

As used herein, an ultralow thermal expansion (ULE) material is materialhaving a coefficient of thermal expansion (in units of 10⁻⁶/° C.) ofless than about 5, 0.5, 0.2, 0.05, 0.02, 0.010, or 0.007. Some ULEmaterials include lithium aluminum silicon oxide glass ceramics such asZERODUR glass ceramic and fused quartz.

As used herein, a circulating optical path is an optical path defined byreflective or refractive optical surfaces such that an optical beam isdirected one or more or multiple times to the optical surfaces. In someexamples, an optical beam propagates multiple times along a commonoptical path, such as an optical path defined by a Fabry-Perot or ringresonator. In another example, an optical beam is reflected by a twomirror system so as to trace an elliptical path on a mirror surface, andthe optical beam can follow different paths in each pass. Such systemsare described in Herriott and Schulte, “Folded Optical Delay Lines,”Applied Optics 4:883-889 (1965), which is incorporated herein byreference. In the examples disclosed below, such circulating opticalpaths are defined using reflective surfaces, but refractive opticalelements can be used as well.

In some examples described herein, a measurement optical beam is dividedinto a probe beam that is directed to a target, and a reference beamthat can be used for calibration by being directed to a reference lengthor serve as a local oscillator beam and used for heterodyne detectionand target distance estimation in combination with the probe beam. Inother examples, a beam directed to a target is referred to as ameasurement beam. In the disclosed examples, portions of one or moreoptical beams are directed to a target, detectors, or communicated fromone to one or more destinations. As used herein, a beam portion refersto any fraction of an optical beam, including the entire optical beam.

In some examples, rotations are described with reference to azimuthalangles and elevational angles. While such angles are typically definedwith respect to vertical and horizontal axes, as used herein,orientation with vertical and horizontal is not required. Typically,systems are described with reference to such angles with systems assumedto be in a standard in-use orientation. For purposes of convenientillustration, corner cubes are shown in some figures as roof prisms.

Swept Frequency Laser Radar

In the following, various configurations and aspects of laser radarsystems are disclosed. The disclosed systems, system components,modules, and associated methods can be used in various laser radarsystems. In typical examples, so-called swept frequency laser radarsystems are provided. Typical coherent radar systems generally use oneor more laser diode light sources. The laser diode frequency is directlymodulated by modulating a laser diode injection current or modulatinglaser diode temperature or in some other way. The laser frequency isgenerally modulated with a waveform so as to produce a linear frequencysweep or linear “chirp.” Laser frequency f(t) can then be expressed as afunction of time t as:f(t)=f ₀+(Δf/Δt)t=f ₀ +βt,wherein f₀ is a laser initial frequency and β=Δf/Δt is a rate of laserfrequency change. Linear sweeps are not required and arbitrary laserfrequency variations as a function of time are theoretically useful suchas stepped or other discontinuous frequency variations, or continuousvariations based on polynomial or other functions, but linear chirps aregenerally more convenient and practical. A frequency modulated (FM)measurement beam is focused at a target, and a portion of the beam isscattered, reflected, refracted or otherwise directed so as to becollected by receiver optics. A local oscillator beam (“LO beam”) isgenerally obtained as a portion of the same laser beam used to producethe measurement beam. A round trip transit time associated withmeasurement beam propagation to and from the target results in afrequency difference between the returned portion of the measurementbeam (the return beam) and the local oscillator. This frequencydifference can be used to determine target distance. The return beam andthe LO are directed to a detector such as a PIN photodiode (typicallyreferred to as a square law detector) to produce sum and differencefrequency signals. The sum frequency (at a several hundred THz for a 1.5μm measurement beam) is beyond available detector bandwidth, but thereturn and LO beams also produce a difference frequency Δf (heterodynefrequency) within the detector bandwidth. A distance R to a targetlocation can be calculated as R=cΔf/2β, wherein Δf is the heterodynefrequency associated with the return beam, β is the chirp rate, and c isthe speed of light. Heterodyne frequency generation also requires thatthe LO and return beam are not orthogonally polarized, but since rangeis determined based on frequency differences and not amplitudes,polarization effects generally do not degrade laser radar performance.

Successful laser radar systems control or measure laser frequencyprecisely as the accuracy of range measurements can be limited by thelinearity of laser frequency modulation. For example, if a target is onemeter distant, a linearity of one part per thousand is necessary toensure 1 mm accuracy. Accordingly, laser sources for FM laser radar areconfigured to provide highly linear chirps, and variances from linearityare detected and compensated. In some cases, range measurements can haveprecisions in the few micron range.

FM laser radar systems are largely immune to ambient lighting conditionsand changes in surface reflectivity because signal detection is based onheterodyne beat frequency, which is independent of signal amplitude andunaffected by stray radiation. Thus, amplitude or intensity variationsin the return beam, the measurement beam, or the LO beam tend to havelittle effect on range measurements. In addition, coherent heterodynedetection can successfully detect optical signals to the shot noiselimit so that FM coherent laser radars can make reliable measurementswith as little as one picowatt of return beam power, corresponding to anine order-of-magnitude dynamic range.

A representative optical fiber based laser radar is illustrated in FIG.51A. As shown in FIG. 51A, polarization retaining fibers are used forbeam coupling and delivery, but in other examples, free spacepropagation, surface waveguides, or other optical systems can be used. Alaser sweep controller 5102 is coupled to a first laser diode 5104 and asecond laser diode 5106 so as to produce chirped laser beams that arecoupled to optical fibers 5108, 5110, respectively. A 3 by 2 fibercoupler 5112 receives these beams and couples portions of each beam to ameasurement path optical fiber 5114 and a reference path optical fiber5116. The dual wavelength beam is delivered by an optical fiber 5114 toa laser radar optical system 5120 that includes a reflector 5122 and abeam shaping lens 5124. A scanning stage 5130 is configured to provideazimuthal and elevational rotations so as to direct a measurement beam5132 to a target 5134. Portions of the measurement beam 5132 arereturned by the target 5134 and coupled into the optical fiber 5114. Thereflector 5122 reflects a portion of the combined beams from the opticalfiber 5114 back into the optical fiber 5114 to produce local oscillatorbeams associated with each of the laser diodes 5104, 5106.

To determine target range, the return beam and local oscillator beamspropagate to the combiner 5112 and a portion of these combined beams isdirected to at least one measurement photodetector 5140. Heterodynefrequencies associated with both laser diodes are produced, and a signalprocessor 5142 determines range estimates based on the heterodynefrequencies and laser chirp rates.

Range calibration can be provided based on portions of the laser beamsthat propagate in the optical fiber 5116 to a 2 by 2 fiber coupler 5150that in turn couples beam portions to a fiber 5154 that is coupled to alength reference 5156. The coupler 5150 also directs beam portions to afiber 5158 to produce local oscillator beams based on reflection of thecombined beams at a fiber end 5160. Beams associated with the lengthreference are combined with the local oscillator beams at a referencephotodetector 5162. Heterodyne frequencies associated with the lengthreference are produced and detected by the signal processor 5142 for usein calibration. For example, if the length reference has a length L andproduces a heterodyne frequency difference Δf, a range scale factorR_(L) can be obtained as R_(L)=L/Δf. Target range can then be calculatedas R=R_(L) Δf_(m).

Reference standards can be based on optical fibers having a preciselength and that are coated with a metallic or other coating to preventlength changes associated with ambient humidity. Typical referencelengths are in a range of from about 1 m to 5 m. Temperature dependentlength changes can be controlled by retaining the fiber in a temperaturecontrolled, hermetically sealed container, or monitoring temperature toestimate temperature dependent length changes. Such a container can bemade from aluminum or copper with fiber inputs and outputs metalized andsoldered to the container where they pass through the wall. An O-ringseal can be incorporated into the lid to complete the sealing. Sealingcan also be accomplished by welding or soldering the container. Inaddition, the container can be backfilled with a dry, inert gas toprovide a moisture free environment for the fiber. A reference fiber canbe overcoated with a polyimide layer and a sealing coat to reducehumidity induced changes. Sealing coatings can include metals such asgold or can be of inert materials such as carbon. Other representativereference standards based on multiple reflections are described in theexamples below.

A controller 5128 is coupled to the sweep controller 5102, the signalprocessor 5142, and the scanning stage 5130 to permit assessment ofrange over target areas. The scanning stage 5130 can be configured toscan in a raster, a W-pattern, a spiral, or other selected pattern. Inaddition, the controller 5128 can be used to select and/or vary laserdiode sweep rates based on target range, as well as measure and controlcomponent temperatures, but temperature sensors and controls are notshown in FIG. 51A. Typically optical isolators are inserted at variouslocations to reduce or eliminate unwanted back reflections that canproduce undesirable changes in laser operation, but for convenientillustration, optical isolators are not shown in FIG. 51A. Laser beamwavelengths can be selected as convenient, and wavelengths of betweenabout 1000 nm and 1600 nm are typical. A visible beam can be included aswell to permit visual identification of a measurement location, but isnot shown in FIG. 51A.

FIG. 51B depicts linear frequency modulations (chirps) of measurementbeam portions returned from an arbitrary moving target location fromfirst and second laser diodes modulated to have different chirp rates.The chirps of the associated local oscillator beams are also shown.Heterodyne frequencies f₁, f₂ associated with each laser diodes areshown. The up and down chirped frequency differences for each laserinclude a Doppler shift contribution f_(d) such that f_(1up)=f₁−f_(d),f_(1down)=f₁+f_(d), f_(2up)=f₂−f_(d), and f_(2down)=f₂+f_(d). Based onthese difference frequencies, target speed and target range can beestimated. Typically, heterodyne frequencies are selected to be between1 MHz and 100 MHz, but other frequencies can be used.

If a target surface is moving relative to the measurement beams,heterodyne frequencies corresponding to laser frequency upsweeps will bedifferent from heterodyne frequencies corresponding to frequencydownsweeps due to Doppler frequency shifting. Measurement of frequencydifferences between upsweeps and downsweeps permits estimation of targetspeed as well as target range. Any configuration of frequency sweepsthat includes local oscillator frequencies that are greater and lessthan the measurement beam frequencies can be used. Additional detailsconcerning such determinations can be found in Rezk and Slotwinski, U.S.Patent Appl. Publ. 2011/0205523, which is incorporated herein byreference.

Described below are numerous examples of methods, components, systems,and sub-systems for laser radar based range finding, laser radar, andlaser tracking. These examples can be combined with one another to formvarious example laser radar systems, but these examples are not to beused to limit the scope of the disclosure.

Remote Local Oscillators

As discussed above, coherent laser radar systems generally direct aprobe beam to a target, and mix radiation returned from the target witha reference optical signal that is referred to as a local oscillatorsignal. The returned radiation is then detected using interferencebetween the returned radiation and the local oscillator using so-calledcoherent detection. In some cases, then the returned radiation and thelocal oscillator have a common frequency, and coherent detection isreferred to as homodyne detection. In most practical applications, theprobe beam and the local oscillator signal are at different frequencies,and the coherent detection is referred to as heterodyne detection. Inheterodyne detection, signals associated with a sum and a difference ofprobe and local oscillator signals are produced. Due to the very highfrequencies associated with the sum frequency (500 THz or more), onlythe difference frequency is generally detected and processed.

If a local oscillator is provided that does not propagate along a commonpath with the probe beam, range errors can be introduced due to driftsbetween a measurement optical path and a local oscillator optical path.While these drifts can in some cases be compensated, disclosed hereinare local oscillator configurations that provide a more common path withthe probe beam so that variations in the probe beam path and the localoscillator path tend to be similar and can offset each other. Thisreduces or eliminates errors due to temperature, vibration pressure,humidity or other environmental effects. Fiber-optic systems thatdeliver a dual wavelength measurement beam to scanning optics such as apentamirror over a common optical fiber also reduce any dependencies onenvironmental effects. Thus, the disclosed systems are well adapted tofiber beam delivery to moving portions of a laser radar, i.e., fiberscan deliver beams to optical systems mounted to rotational stages orother moving parts without introducing measurement error.

In representative examples described below, division of wavefront ordivision of amplitude approaches are used to form a measurement beam andan LO beam from a single input beam. In some examples, an input beam isdirected through a first surface of a catadioptric optical system to asecond surface of the catadioptric optical system that reflects aportion of the input beam back to the first surface. The first andsecond surfaces typically have reflective and transmissive areas so asto produce a first beam by transmission without reflection, and a secondbeam based on reflection by the first and second surfaces beforetransmission through the second surface. The first and second surfacescan be air spaced, or be provided as surfaces on a solid catadioptricoptical element. Divergences of the first and second beams can beselected based on surface curvatures, spacings, and refractive indices.For convenience, a more diverging beam of the first and second beams istypically referred to as a measurement beam and a lesser diverging beam(typically collimated) as an LO beam. A reflective area on the secondsurface can be a central area or can be situated at a surface perimeter.In some configurations, a source location is typically relayed by theoptical system to a different location such as closer to or more distantfrom a focus adjustment corner cube that is used to adjust beam focus ona target. Closer source locations typically permit use of smaller focusadjustment corner cubes, while more distant source locations tend toreduce the magnitude of any ghost reflections from corner cube surfaces.Input fibers can be index matched at fiber output surfaces to reducereflections, or fiber output surfaces can be tilted with respect to apropagation axis defined by the fiber. In some examples, folded opticalsystems are used in which an LO beam, a measurement beam, or both aredirected along a folded path. Mirrors, prisms, or other reflectiveoptical elements can be used to fold the path as may be convenient.

Remote Local Oscillator Example 1

With reference to FIG. 1, a laser radar system 100 includes an opticalfiber 102 coupled to a light source such as a swept frequency laser thatdirects a chirped optical beam to a beam splitter assembly 104. A lens106 is situated to receive the optical beam from the optical fiber 102and produce a collimated beam that is directed into a beam splitter 108.The beam splitter 108 includes a partially reflective surface 109 thatdirects a measurement portion of the optical beam along a measurementbeam path to an optical filter or waveplate 110 and an additional beamshaping lens 112. In some examples, the beam splitter 108 is apolarizing beam splitter but a non-polarizing beam splitter can be usedas well. As shown, a mirror 114 is situated so as to direct ameasurement beam 116 to a focus adjustment corner cube 120. Themeasurement beam is reflected by the corner cube 120 so as to bereflected back into the focus adjustment corner cube 120 by a returnreflector 119. Upon exiting the focus adjustment corner cube 120, themeasurement beam is shaped or focused by a lens 115 to form an opticalbeam 117 that is directed to a target. For clarity in FIG. 1, themeasurement beam path in the corner cube is not shown, but the returnreflector is generally displaced from a measurement beam propagationaxis so that the measurement beam is not obstructed. By displacing thecorner cube 120 from the lens 115 with one or more translation stages(not shown in FIG. 1), the measurement beam can be focused at a selectedtarget distance. Portions of the measurement beam returned from thetarget (“return beam”) propagate in the opposite direction of themeasurement beam and are transmitted by the beam splitter 108 to one ormore filters or waveplates 130 and a detector focus lens 132 so that thereturn beam is received by a detector 134.

The partially reflective surface 109 of the beam splitter 108 transmitsa local oscillator portion (LO beam) to an optical filter or waveplate118 and then to the focus adjustment corner cube 120. The LO beam isdirected by the focus adjustment corner cube 120 to an LO reflectingcorner cube 122 along a path 125. The LO corner cube 122 is situated sothat the LO beam is returned to the focus adjustment corner cube 120 andpropagates along a path 127 that is parallel to and displaced from thepath 125. In other examples, a roof prism is used to reflect and shiftthe LO beam instead of a corner cube. The LO beam then exits the focusadjustment corner cube 120 and is incident to a retroreflector 124 thatredirects the LO beam along the paths 127, 125 so as to return to thebeam splitter 108. The beam splitter 108 reflects at least a portion ofthe LO beam so as to be incident to the detector 134. Thus, portions ofboth the measurement beam and the LO beam are received by the detectorso that a heterodyne signal can be obtained.

As shown in FIG. 1, the LO beam propagates through the corner cube fourtimes along optical paths that are parallel to and displaced frommeasurement beam paths. The measurement beam propagates twice throughthe focus adjustment corner cube 120 before being directed to a target.The return beam from the target follows the measurement beam path inreverse thus traversing the focus adjustment corner cube 120 two moretimes so that any path differences produced by the focus adjustmentcorner cube 120 are substantially the same for both the LO beam and themeasurement/return beam.

Although the focus adjustment corner cube 120 of FIG. 1 is also used inproviding the LO beam, the size of the focus adjustment corner cube 120need not increase to accommodate LO optics, depending on the location ofthe LO beam path. In the example of FIG. 1, the input/exit surfaces ofthe corner cubes 120, 122 and other optical surfaces can produceundesirable reflections that can complicate range determination. Eachsuch reflection typically produces a secondary or “ghost” LO beam thatmay be used (either intentionally or accidentally) in determining arange estimate. Since the locations of these surface reflections areknown, multiple measurements based on a primary LO beam or on one ormore secondary “ghost” beams can be combined to produce range estimates.However, in most practical examples, these ghost reflections are avoidedto permit simpler range determinations. To reduce the magnitude of suchreflections, some or all surfaces can be provided with suitableantireflection coatings. Alternatively, surfaces can be tilted withrespect to the local oscillator beam such as by providing wedged opticalelements. In other examples, hollow optical elements can be used toavoid such surfaces. For example, a hollow corner cube can be formed ofthree first surface mirrors. Such a hollow corner cube lacks surfacesassociated with unwanted multiple reflections.

Remote Local Oscillator Example 2

With reference to FIG. 2, a laser radar system includes an optical fiber202 that has an exit surface 204 configured to deliver a chirped laserbeam 206 to a collimating lens 208 and to a beam splitter cube 210. AnLO portion of the chirped laser beam 206 is reflected to mirrors 212A,212B and then to a focus adjustment corner cube 220. The focusadjustment corner cube 220 is generally configured to be translatable toprovide focus adjustment for a measurement portion of the chirped laserbeam, but translation mechanisms are not shown in FIG. 2. The LO beampropagates along a path 221 through the corner cube 220 to an LO rightangle prism 222 and to a retroreflector 224. The LO right angle prism222 is situated so as to serve as a roof prism, as the prism hypotenuseis used as an entrance/exit surface, and the right angles faces serve toreflect the LO beam. Other prisms, such as roof prisms can be used. Theretroreflector 224 returns the LO beam along the path 221 to the rightangle prism pair 212A, 212B and to the beam splitter cube 210. As shownin FIG. 2, the beam splitter cube 210 transmits the LO beam or a portionthereof to a focusing lens 231 that directs the LO beam to an opticalfiber 232 that can be coupled to a detector (not shown).

The measurement portion of the chirped laser beam 206 (the measurementbeam) is transmitted by the beam splitter cube 210 and focused by a lens227 to form a diverging beam at or near a mirror 225 that directs themeasurement beam to the focus adjustment corner cube 220 along ameasurement beam path to a return reflector 230. The measurement beampath is offset from the LO beam path 221, but is not indicated in FIG. 2for clarity. After reflection by the return reflector 230, themeasurement beam is reflected back along the measurement beam pathtowards the mirror 225. Because the measurement beam is diverging, themirror 225 does not appreciably obstruct the measurement beam. Themeasurement beam is incident on an objective lens (not shown) thatshapes or focuses the measurement beam at a target surface. A returnbeam from the target follows the measurement beam path in reverse sothat the beam splitter cube 210 reflects the measurement beam to thefocusing lens 231 that directs the return beam (and the LO beam) to theoptical fiber 232.

As shown in FIG. 2, the LO beam and the measurement beam/return beampropagate along parallel, displaced paths in the focus adjustment cornercube 220, and the total path length in the focus adjustment corner cube220 is substantially the same for the LO beam and the measurementbeam/return beam. The LO beam propagates as a collimated beam in thefocus adjustment corner cube 220, while the measurement beam and returnbeam are diverging and converging respectively.

Remote Local Oscillator Example 3

In some examples, a measurement beam portion and an LO beam portion areconfigured to propagate at a slight angle with respect to each other.Referring to FIG. 3, a portion an optical system 300 for a laser radarincludes an optical fiber 302 that extends through a ferrule 304 that isretained by a supporting substrate 314. The optical fiber 302 emits achirped optical beam from a fiber end surface 303 that is directed to aFresnel zone plate 305 or other optical element that transmits ameasurement beam portion 306 along an axis 301. The measurement beam 306typically retains a beam divergence corresponding to a numericalaperture of the optical fiber 302. An LO beam portion is collimated bythe zone plate 305 so as to propagate along an axis 308 that is at anangle with respect to the axis 301. The LO beam is coupled through afocus adjustment corner cube 320 to a reflective surface 316 thatreturns the LO beam toward the Fresnel zone plate 305 but along an axis309 that is at an angle with respect to the axes 301, 308. The LO beamis focused by the zone plate 305 at a location displaced from butapproximately or exactly coplanar with an emission area of the fiber endsurface 303. In some examples, the LO beam is focused onto a polishedferrule surface 330 that reflects the LO beam along an axis 311. The LObeam then propagates back to the reflective surface 316 through thefocus adjustment corner cube 320 and returns along an axis 311 so thatthe zone plate 305 focuses the LO beam back into the fiber 302. The zoneplate 305 reimages the end surface 303 onto the end surface 303. Becausethe end surface 303 is situated at or near a focal point of the zoneplate 305, LO beam propagation along the axes 308, 309, 310, 311 resultsin refocusing of the LO beam into the optical fiber 302.

The measurement beam 306 is divergent and is directed through the cornercube 320 to a return reflector and then back through the corner cube 320to an object lens that focuses the measurement beam at a target. Areturn beam follows this path in reverse, and is coupled into theoptical fiber 302 along with the LO beam. To simplify FIG. 3, themeasurement beam path, return reflector, and objective lens are notshown. The divergence of the measurement beam and the relative tilt ofthe LO propagation axes with respect to the axis 301 are exaggerated forconvenient illustration. Multiple reflections at corner cube surfacescan be reduced with anti-reflection coatings, and effects associatedwith unwanted reflections at the end surface 303 of the fiber 302 can bereduced using an angle polished (APC) connectorized fiber.

Remote Local Oscillator Example 4—Measurement Beam/LO Beam Production

Collimated beam portions (typically used as LO beams) and diverging beamportions (typically used as measurement beams) can be obtained with avariety of optical arrangements, examples of which are illustratedbelow. Such optical systems can be based on division of beam amplitudeor wavefront as may be convenient. In some examples, optical fibers areused to supply a beam to be divided into a measurement beam and an LObeam. To reduce reflections from fiber end surfaces, curved or angledsurfaces can be used, or fiber end surfaces can be secured with anoptical adhesive that can be selected to provide an approximate indexmatch. In some examples, the fiber is cemented to a first surface, andLO and measurement beams are formed using combinations of reflection andtransmissive at the first surface and a second surface. Beam collimationor divergence can be provided based on surface curvatures andseparations.

Reflective Division of Wavefront Coupled Fiber Configurations

In the examples of FIGS. 4-7, a fiber is coupled to a catadioptric orother optical element, typically with an optical adhesive to reducefiber end surface reflections. With reference to FIG. 4, an opticalfiber 402 is configured to emit an optical beam that is transmitted bytransmissive aperture 404 defined on an first surface 403 of a doubleconvex lens 405 formed of an optical glass, fused silica, or othertransmissive optical material. The optical fiber 402 can be cemented tothe first surface 403. A portion the optical beam is reflected by acentral reflective area 410 defined on a second surface 408 of the lens405 to an outer reflective area 406 on the first surface 403. Thereflective area 406 directs the reflected beam to an outer transmissiveportion 409 of the second surface 408 so that a beam 412 is formed. Aportion of the input optical beam is transmitted by the outertransmissive portion 408 of the second surface without internalreflections to form a diverging beam 414. The beams 412, 414 can befurther directed so as to become a measurement beam or an LO beam, andbeam shapes, sizes, and divergences can be selected based on curvaturesand spacing of the first and second surfaces 403, 408 and an index ofrefraction of the lens 405. The lens 405 need not be biconvex, but canhave convex, concave, spherical, or aspheric surfaces as illustrated insome of the following additional examples. In addition, the first andsecond surfaces 403, 408 can be defined on separate surfaces and spacedapart and a solid lens element is not required.

FIG. 5 illustrates an optical system that produces LO and measurementbeams from a single beam input received from a fiber 502 at atransmissive aperture 504 at a first surface 507 of a catadioptricoptical element 505. An LO beam is obtained by reflecting a portion ofthe input beam at a central reflective area 510 of a second surface 508and an outer reflective portion 506 of the first surface 507. The LObeam is then refracted at an outer portion of the second surface 508 toproduce a collimated beam 512. A diverging measurement beam is producedby transmitting a portion of the input beam around the centralreflective area 510 of the second surface 508 to form a measurement beam514. The catadioptric optical element 505 includes a convex firstsurface and a concave second surface, but other surface curvatures canbe used.

In another example illustrated in FIG. 6, an optical system produces LOand measurement beams from a single beam input received from a fiber 602at a transmissive aperture 604 at a first surface 607 of a catadioptricoptical element 605. A measurement beam is obtained by reflecting aportion of the input beam at a central reflective area 610 of a secondsurface 608 and an outer reflective portion 606 of the first surface607. This portion of the input beam is then refracted at an outerportion of the second surface 608 to produce a measurement beam 614 thatdiverges from a focus 620. A collimated LO beam is produced bytransmitting a portion of the input beam around the central reflectivearea 610 of the second surface 608 to form an LO beam 612. Thecatadioptric optical element 605 includes convex first and secondsurfaces, but other surface curvatures can be used. A curvature of thesecond surface is selected so that the LO beam is substantiallycollimated.

FIG. 7 illustrates yet another example optical system. In this example,an optical fiber 702 delivers an input beam to a transmissive aperture704 defined on a first surface 707 of a catadioptric optical element705. An LO beam portion is transmitted around a central reflective area710 of a second surface 708 to a lens 711 that collimates the LO beam.The central reflective area 710 can be provided as a reflective coatingon one or both of the lens 711 and the catadioptric optical element 705.A measurement beam is obtained by reflecting a portion of the input beamat the central reflective area 710 of the second surface 708 and anouter reflective portion 706 of the first surface 707. The measurementbeam is then refracted at a convex surface 713 of the lens to form adiverging measurement beam 714. In this example, the first surface 707and the second surface 708 are planar surfaces, and the additional lenselement (lens 711) is a plano-convex lens that can be cemented to thecatadioptric optical element 705.

Examples in which fiber end surfaces are tilted with respect to a fiberaxis are shown in FIGS. 8-9. Referring to FIG. 8, a fiber 802 isinserted into an aperture defined in a first reflective optical element806. To reduce back reflections, the fiber 802 terminates at a tiltedend surface 804. As shown in FIG. 8, the tilted end surface 804 issituated so that a fiber axis 801 aligns with an optical system axis 803in consideration of refraction of the fiber axis 801 upon exiting thefiber 802. In other examples, the fiber is retained so that the axes801, 803 form a straight line axis, and emission from the tilted endsurface 804 propagates at an angle with respect to the optical systemaxis 803. In most cases, these tilts are small, and are exaggerated inFIG. 8 for purposes of illustration. An LO portion of an input beam fromthe fiber 802 is reflected by a central reflective area 816 defined on asurface 812 of a convex/concave catadioptric optical element 808 back toa reflective surface 807 of the reflective optical element 806. Thereflected portion is then refracted by a convex/concave catadioptricoptical element 808 to form a collimated beam 820. A measurement beamportion is transmitted to the catadioptric optical element 808 toproduce a measurement beam 818. The optical elements 806, 808 are spacedapart, and spacing and curvatures can be selected for beam shaping aspreferred.

Referring to FIG. 9, a fiber 902 is inserted into an aperture defined ina planar reflective optical element 906. To reduce back reflections, thefiber 902 terminates at a tilted end surface 904. The effects of ameasurement portion of an input beam from the fiber 902 is reflected bya central reflective area 916 of a surface 912 of a plano-convexcatadioptric optical element 908 back to a reflective surface 907 of thereflective optical element 906. The reflected portion is then refractedby the catadioptric optical element 908 to form a diverging beam 918. AnLO beam portion is transmitted to the catadioptric optical element 908to produce an LO beam 920. The optical elements 906, 908 are spacedapart, and spacing and curvatures can be selected for beam shaping aspreferred.

FIG. 10 illustrates a configuration similar to that of FIG. 7, but witha folded axis. A fiber 1002 is situated to couple an input beam to acube reflector 1004 formed as a right angle prism pair with a reflectivecoating through a transmissive aperture 1007 of a cube reflector inputsurface 1005. A reflective surface 1009 of the cube reflector 1004reflects an LO beam portion around a reflective area 1010 of a cubereflector exit surface 1008 to a plano-convex lens 1014 so as to form acollimated LO beam 1022. A measurement beam is formed by reflecting aportion of the input beam at the reflective central area 1010 and areflective portion 1006 of the cube reflector input surface 1005. Themeasurement beam is then transmitted around the central reflective area1010 to the lens 1014 to form a diverging measurement beam 1020.

In the disclosed examples, solid catadioptric elements are used in whichreflective coatings are provided over selected parts of concave, convex,or planar surfaces. Spaced apart optical elements are also used in someexamples. Generally, a central portion of at least one optical surfaceis reflective and a central portion of at least one optical surface istransmissive so that the optical path for at least one beam is folded,but there are many possible variations. In some cases, an input beam istransmitted through a central transmissive portion of an optical elementto form an LO or measurement beam, without reflection. The other beam(LO or measurement beam) is then produced with reflection at an outerportion of the optical element. A representative example is shown inFIG. 11 in which a fiber 1102 couples an input beam to a solidcatadioptric element 1105 that has a central transmissive area 1110 onan output surface 1100. An LO beam 1116 is formed by transmission andrefraction of a portion of the input beam by the output surface 1100,without reflection. A portion of the input beam is reflected at an outerreflective area 1108 of the output surface 1106 to a reflective inputsurface 1104. The reflected portion forms a focus at 1120 and is thentransmitted by the central transmissive area to form a divergingmeasurement beam 1124.

FIG. 12 illustrates a solid catadioptric optical element 1208 that issituated to receive an input optical beam exiting an optical fiber 1202at a tilted end surface 1204. The optical element 1208 includes a firstsurface 1210 and a second surface 1220 that are centered on an axis AX.The first surface 1210 includes an outer reflective area 1212 and acentral transmissive area 1214. The second surface 1220 includes acentral transmissive area 1224, an intermediate reflective area 1226,and an outer transmissive area 1230. As shown in FIG. 12, a collimatedbeam 1240 is formed. A diverging beam (not shown) can also be formed bytransmission of a portion of the input beam through the centraltransmissive area 1224. Because of the tilt of the fiber end surface1204, a reflected beam 1250 is directed away from the axis AX and is notcaptured by the optical system.

As shown in the examples above, collimated LO beam generation isaccompanied by generation of a non-collimated, diverging or convergingmeasurement beam. A so-called “two power” element can be used as shownin FIG. 13. A solid catadioptric element 1304 has a first surface 1318having a transmissive central aperture 1322 and reflective annulus 1320.A second surface 1312 has a central reflective area 1316 and atransmissive annulus 1314. Curvatures of the first and second surfacescan be selected so that the catadioptric element 1304 has a first poweror focal length for a straight through beam and second power for a beamthat is reflected along a folded path within the catadioptric opticalelement 1304. For a meniscus shape, if the curvatures of the twosurfaces are approximately equal, the catadioptric element serves as azero power lens for the straight through beam. (In this case curvaturesare selected based on thickness, as equal curvatures produce zero poweronly in a thin lens approximation.) Production of a collimated beambased on an input beam from an optical fiber is illustrated, and adiverging beam is not shown.

With reference to FIG. 14, an optical fiber 1402 is situated to directan input optical beam 1404 to a collimating lens 1408 that produces acollimated beam 1412 that is directed to a reflective surface 1417 of aright angle prism 1416. The prism 1416 includes a planar reflective area1420 and a curved reflective area 1424. The planar reflective area 1420is situated to reflect an outer portion of the collimated beam 1412 toproduce a collimated LO beam 1428. The curved reflective area 1424 issituated to focus an inner portion 1432 of the collimated beam 1412 to afocus 1436 and produce a divergent measurement beam 1440. If the centralreflective area 1424 is concave, the measurement beam comes to a realfocus 1436 as shown. However a convex curvature may be used and wouldproduce a virtual focus. The central area can also be planar, and theouter area curved. Generally, any combination of different curvaturesfor inner and outer portions can be used and the incident beam 1412 neednot be collimated. A right angle reflection is shown for convenientillustration, but other angles can be used. A reflective surface 1410 atthe fiber exit surface can be provided if multiple reflections of an LObeam are desired.

Refractive Division of Wavefront Coupled Fiber Configurations

In many examples, reflective surfaces are included, but optical systemsthat include only transmissive or refractive surfaces can be used. Forexample, a refractive optical element having at least one bifurcated orother compound optical surface can be used. As shown in FIG. 15, anoptical system includes a fiber 1502 that is situated to deliver aninput optical beam along an axis 1508 to a double convex lens 1510having convex input/output surfaces 1512, 1514. (A slight deviation ofthe axis 1508 with respect to a propagation axis 1507 in the fiber 1502due to a tilt of a fiber output surface 1503 is not shown.) In theexample of FIG. 15, the output surface 1514 has an outer annular portion1516A and a second lens 1518 is secured to the first lens 1510 so as tocover an axial portion. The second lens 1518 is shown as aconcave/convex lens that can be cemented or otherwise contacted to thefirst lens 1510, or can be air spaced from the first lens either on oroff of the axis 1508. Other surface curvatures can be used for one orboth of the first lens 1508 and the second lens 1518, and output sidecurvature of the first lens 1510 need not match the input side curvatureof the second lens 1518. In other examples, bifurcated lenses are used.

An annular portion 1516 of the lens 1510 is selected to produce aconverging optical beam 1530 that can serve as an LO beam. The secondlens 1518 produces a converging beam 1532 that can serve as ameasurement beam. As shown above in FIG. 3, a reflective surface 1536can be provided at the fiber exit surface 1503 to direct an LO beam sothat after multiple reflections, the LO beam is coupled back into theoptical fiber 1502. In other examples, a collimating lens can be securedto the fiber, and a second lens can be used to intercept a portion ofthe collimated beam to form a measurement beam.

Representative bifurcated lenses 1602, 1612, 1622 are shown in FIGS.16A-16C. The lens 1602 includes a first convex surface 1604 and a secondconvex surface 1606. An axial portion 1608 of the second convex surface1606 is provided with a curvature different from that of a curvature ofother portions of the second convex surface 1606. Similarly, the lens1612 includes a plano surface 1614 and a convex surface 1616. An axialportion 1618 of the surface 1616 is provided with a curvature differentfrom that of a curvature of other portions of the convex surface 1606.In the example of FIG. 16C, a lens 1622 includes a convex surface 1624and a plano surface 1626. An axial portion 1628 of the lens 1622 isprovided with a curvature different from that of a curvature of otherportions of the plano surface 1626. The axial portion 1628 can beintegral to the lens 1624, or can be provided with an additional lenselement secured to or spaced apart from the surface 1626.

Polarization Based Division of Amplitude Beam Separation

Referring to FIG. 17, a beam separation optical system includes anoptical fiber 1702 that is configured to direct an optical beam along anaxis 1704 to a collimating lens 1708 and a waveplate 1710. The waveplate1710 is oriented so as to produce a selected state of polarization (SOP)for the received optical beam. In most practical examples, the opticalbeam from the optical fiber 1702 is a linearly polarized beam, and thewaveplate 1710 is a half wave retarder that can be oriented to produce alinear SOP in an arbitrary direction. An optical assembly 1711 issituated so as to receive the optical beam, and includes an input lens1712, a polarizing beam splitter (PBS) 1714, a quarter wave plate 1719,and a wedge prism 1720 with a reflective surface 1721. A portion of theinput beam in a first SOP is transmitted by a PBS reflecting surface1716 through the quarter wave plate 1719 and is then reflected by thereflective surface 1721 back through the quarter wave plate 1719. Thequarter wave retarder 1719 is arranged so that upon traversing thequarter wave retarder 1719 twice, the first SOP is transformed into asecond SOP, orthogonal to the first SOP. The reflecting surface 1721thus directs the transmitted beam to the PBS reflective surface 1716which reflects the beam to an output lens 1734 so as to produce acollimated beam 1740.

The PBS reflective surface 1716 directs a measurement beam portion ofthe input beam in the second SOP to a quarter wave retarder 1724 and areflective surface 1728 that can be provided on a surface of a solidprism 1725. The quarter wave retarder 1724 is arranged so that upontraversing the quarter wave retarder 1724 twice, the second SOP istransformed into the first SOP so that the reflected measurement beam istransmitted by the PBS reflective surface 1716. The output lens 1734converges the measurement beam to a focus at 1738 beyond which an outputmeasurement beam 1741 diverges. The SOP of the input beam can beselected based on retardance and orientation of the wave plate 1710, sothat portions of the input beam reflected and transmitted by the PBS canbe chosen to obtain a selected division of optical power betweenmeasurement and LO beams. As shown in FIG. 17, the beams 1740, 1741propagate along non-parallel axes.

The collimated LO beam 1740 is directed to a focus adjustment cornercube and return mirror (not shown in FIG. 17) and is returned to thelens 1734. However, the returned LO beam is incident to the reflectivesurface 1716 so as to be directed to reflective surface 1722 withouttraversing a waveplate. In contrast to the reflective surface 1721, thereflective surface 1722 does not have a corresponding retarder such asquarter-wave retarder 1719. The reflective surface 1722 then directs theLO beam back to the focus adjustment corner cube and return mirror. TheLO beam is then returned to through the lens 1734 so as to be reflectedby the reflective surface 1716 to the quarter-wave retarder 1719 forreflection by the reflective surface 1721 to the fiber 1702. Typically,tilts of the reflective surfaces 1720, 1721, 1728 are chosen to be assmall as possible while maintaining sufficient separation of the LO beamand the measurement beam, and also to provide sufficient displacement ofthe LO beam at reflective surfaces 1721, 1722.

With reference to FIG. 18, an optical fiber or other input beam sourceis situated to direct an optical beam along an axis 1802. A first beamshaping lens 1806 and a polarizing beam splitter (PBS) 1810 are situatedon the axis 1802, and a reflective surface 1814 of the PBS 1810 issituated to reflect a portion of the input beam along a folded axis 1816and transmit a portion of the input beam to a quarter wave retarder1818. The reflected and transmitted beam portions correspond to firstand second orthogonal (typically linear) states of polarization (SOPs),respectively. A second beam shaping lens 1822 is situated along the axis1802 to produce a measurement beam that can be directed to a target.(Alternatively, this beam can serve as an LO beam.) The quarter waveretarder 1818 is situated to transform the second SOP that is associatedwith the transmitted beam into a circularly polarized transmitted beam.This circularly polarized beam is directed to a target as a measurementbeam, typically using a focus adjustment corner cube arrangement such asillustrated in FIG. 1.

A portion of the measurement beam reflected at a target can be returnedto the second beam shaping lens 1822 as a circular polarization with orwithout a change in handedness. If returned with a change in handedness,the circular SOP of the returned beam is converted by the quarter waveretarder 1818 into the second SOP that is reflected by the PBS 1810. Athird beam shaping lens 1826 directs the returned measurement beam fromthe PBS 1810 to a detector or to a fiber or other optical system fordelivery to a detector.

The PBS 1810 is situated to direct a reflected beam portion as a first(linear) polarization for use as an LO beam along the folded axis 1816.A quarter wave retarder 1828 is situated on the axis 1816 along with aPBS 1830. A reflective surface 1834 of the PBS 1830 is configured toreflect a beam portion along an axis 1836 that is parallel to and offsetfrom the axis 1802. The quarter wave retarder 1828 is oriented so as toproduce a circular SOP from a linear SOP (the first SOP) as reflected bythe PBS 1810, so that some optical power exits the PBS 1830 along theaxis 1816 and is not recaptured. An LO beam is reflected by the PBS 1830through the quarter wave retarder 1818 to produce an LO beam in a firstcircular SOP. This circularly polarized LO beam is then directed to afocus adjustment corner cube and an LO return reflector 1835 thatdirects the LO beam back along the axis 1836.

FIG. 19 illustrates the optical system of FIG. 18, showing an LO beam1950 in a first circular SOP (for example, as a right handed circularSOP or “RHC”) propagating to a focus adjustment corner cube. Acircularly polarized LO beam 1952 in a second circular SOP (for example,as a left handed circular SOP or “LHC”) orthogonal to the first circularSOP is reflected back to the quarter wave retarder 1818. The reflectedLO beam 1952 is generally returned from a focus adjustment corner cubeand a return mirror or a dedicated LO reflector as shown in FIG. 1, butsuch an LO return reflector is omitted from FIGS. 18-19. Upontransmission by the quarter wave retarder 1818 the returned LO beam 1952is in the first linear SOP that is transmitted by the PBS 1830. Thereturn retroreflector 1835 (shown as a corner cube in FIGS. 18-19)reflects the LO beam back through the PBS 1830 and the quarter waveretarder 1818 so as to propagate as a circularly polarized LO beam 1954in the second circular SOP having a handedness opposite to that of thefirst circular SOP, i.e. as LHC.

The LO beam 1954 is directed to a focus adjustment corner cube/returnreflector and is reflected back to the quarter wave retarder 1818 andthe PBS 1830. The LO beam 1954 is transmitted to the focus adjustmentcorner cube in the second circular SOP and returns in the first circularSOP. The quarter wave retarder 1818 converts the first circular SOP tothe first linear SOP so that the LO beam is reflected through the PBS1810 to the third lens 1826 that couples measurement and LO beams to afiber or directly to a detector. LO beam polarization is readilycontrolled to provide appropriate SOPs, but a returned measurement beamportion cannot generally be so readily controlled and additional powerlosses can arise due to SOP mismatches.

FIG. 20 is a perspective view of an optical system that separates ameasurement beam and an LO beam using polarization optics as shown inFIGS. 18-19. An optical assembly 2004 includes first and secondpolarizing beam splitters 2006, 2008 and quarter wave retarders 2010,2012. An input beam is directed along an axis 2014 to produce a straightthrough measurement beam that propagates to a first mirror 2016. An LObeam is produced by reflections in the PBS 2006 and the PBS 2008 so thatan LO beam is directed to a second mirror 2018 along an axis 2023. Afocus adjustment corner cube 2019 and a return reflector 2020 aresituated to receive the measurement beam and reflect the measurementbeam to an objective lens 2021 shown in FIG. 20 as a two element, airspaced lens. The returned measurement beam follows the optical path inreverse to the optical assembly 2004 so as to propagate along an axis2028 for coupling to a fiber or directly to a photodetector.

The LO beam is directed to the focus adjustment corner cube 2019 and anLO return reflector 2034 (or a portion of the return reflector 2020) soas to be returned to the optical assembly 2004. The LO beam is reflectedat a reflective surface 2011 (or by a retroreflector) at the opticalassembly 2004 back to the focus adjustment corner cube 2018 and returnreflector 2034. The reflective surface 2011 is shown in FIG. 20 as asurface of the polarizing beam splitter 2008, but generally aretroreflector is situated on the axis 2023 and can be secured to orair-spaced from the polarizing beam splitter 2008. After this secondtransit of the focus adjustment corner cube 2019, the LO beam isdirected along the folded axis 2028 for coupling to a fiber or directlyto a photodetector. As a result, measurement beam portion and LO beamportions that reach a detector make the same number of transits of thefocus adjustment corner cube 2019, reducing errors introduced by pathdifferences associated with beam focusing. For convenient illustration,additional lenses used to focus beams are not shown, and in mostpractical examples, the measurement beam is focused at or near the firstright mirror 2016 with a lens 2031 so as to be divergent at the lens2021.

Laser Radar Length References

Laser radar systems typically include a reference arm or lengthreference for use in confirming and/or calibrating range measurements.Some laser radars use a swept frequency measurement beam and a sweptfrequency local oscillator beam. A distance to a target is obtainedbased on a difference or heterodyne frequency between a returned portionof the measurement beam and the local oscillator. The returned portionof the measurement beam is at a laser frequency from an earlier time inthe frequency sweep, and target distance can then be estimated as cΔf/β,wherein c is a speed of light, Δf is the heterodyne frequency, and β isa laser frequency sweep rate. FIGS. 21A-21B illustrate opticalfrequencies as a function of time for a measurement beam and an LO beam.The LO beam and measurement beam are produced from a common chirpedinput beam, and a portion of the input beam is diverted and not launchedto a target so as to serve as an LO beam. A target feature at a selecteddistance returns a portion of the measurement beam and the returnedportion and the LO beam are mixed. FIG. 21A illustrates variation ofoptical frequency as a function of time for each beam, and FIG. 21Billustrates a difference frequency that can be used to estimate a targetrange. At some times, the difference frequency varies due to periodicityin the chirp, but during other times, the difference frequency isconstant and is used in range determination. By interrogating areference standard with a known length, a value of laser sweep rate βcan be estimated, and sweep linearity corrected or calibrated.

FIG. 22 illustrates a representative length standard 2200 that include alithium aluminosilicate glass-ceramic tube 2202 having a length selectedfor use as a reference length. Lithium aluminosilicate glass-ceramic isparticularly advantageous due to its low coefficient of thermalexpansion (CTE), typically less than or equal to about 0.2×10⁻⁷/K. Oneexample of this material is commercially available as ZERODUR glassceramic but other glass ceramics with similar CTEs can be used such asCer-Vit or Sitall. The tube 2202 is sealed with mirrors 2204, 2206 thatinclude dielectric coatings 2205, 2207, respectively. The mirrors arealigned so as to be orthogonal to an axis 2209. The tube 2202 and themirrors 2204, 2206 form a Fabry-Perot resonator and the reflectivity ofthe dielectric coatings 2205, 2207 can be selected to provide a selectedresonator finesse. Typically, reflectances greater than 50%, 75%, or 90%are used. In one example, a resonator length is about 25 cm so thatreflections associated with round trips correspond to 50 cm separations.

An optical fiber 2208 is situated so as to deliver a calibration opticalbeam (typically all or a portion of a measurement or probe beam) to acollimating lens 2210 that directs a collimated beam 2212 along theresonator axis 2209. A focusing lens 2211 is situated to direct thecalibration beam into a detector. The two mirrors form a Fabry-Perotinterferometer. With each pass some light escapes the tube and isfocused by a second lens onto a photodetector. Depending upon thereflectivity of the partial mirrors, multiple heterodyne frequencysignals corresponding to different numbers of passes through the tubecan be generated. For a tube 25 cm in length, each signal represents arange difference of 50 cm. Any of these signals can be used as thereference arm signal. An amplifier 2226 couples the heterodyne frequencysignals to a heterodyne frequency detector 2230 that identifies one ormore heterodyne frequencies associated with a single, one way transit ofthe resonator path and/or multiple round trips. A processor or othermeasurement system 2234 receives one or more identified heterodynefrequencies and determines a scale calibration R_(L) for some or allfrequencies and their associated path lengths. While the glass ceramictube 2202 is made of a very low CTE material, a thermal sensing/controlsystem 2240 can be configured to monitor temperature with one or moretemperature sensors 2242 and heat or cool the tube 2202 with aheater/cooler 2244. In some examples, tube temperature is measured sothat a suitable correction to an effective tube length can bedetermined, and the scale calibration R_(L) is based on the temperaturedependent tube length.

Another representative reference length is illustrated in FIG. 23. Aglass ceramic frame 2302 is provided with reflective surfaces 2302A,2302B or a plurality of reflecting regions or discrete reflectorssituated to reflect an input calibration beam. A laser source 2312 iscoupled to a fiber coupler 2314 that directs a portion of an input beamalong a fiber 2315 to a collimating lens 2316. The collimating lens 2316produces a calibration beam that propagates along a multiply folded path2320 to a retroreflector 2324. The retroflector 2324 is arranged so asto reflect the calibration beam back along the path 2320 to thecollimating lens 2316 to the fiber coupler 2314. A fiber output surface2318 of the fiber 2315 is configured to reflect a portion of the inputbeam, typically about 4%, back towards the fiber coupler 2314 as well.The fiber coupler 2314 delivers the multiply reflected calibration beamfrom the frame 2302 to a detector 2328 as well as the portion reflectedat the fiber output surface 2318. The combined beams produce aheterodyne signal at a photodetector 2328 that can be buffered oramplified with an amplifier 2336 prior to delivery to a calibrationsystem such as illustrated in FIG. 22. Temperature control and/ormonitoring can be provided, and the frame 2302 is typically configuredto be sealed within a container. In one example, cover plates aresecured to the frame so that optical propagation along the optical path2320 can be controlled based on a stable environment.

In other examples, calibration optical paths can be based on otherinterferometer paths such as Mach-Zehnder paths in which a pathdifference is defined using a glass ceramic or other ultrastablematerial. Fabry-Perot etalons do not require a tube, but can be definedby reflectors that are spaced apart by glass ceramic rods or plates.Ring resonator configurations can also be used. Some additional examplesare illustrated in FIGS. 24-25. Referring to FIG. 24, a reference lengthis defined by beam splitters 2404, 2420 and reflectors 2406, 2408, 2410,2412, 2413, 2414, 2416, 2418 that are arranged to direct a first portionof an input beam along an axis 2401 and a second portion along amultiply folded axis 2403. The beam splitters and reflectors can besecured to a temperature stable base of a material such as a lithiumaluminosilicate glass-ceramic. Referring to FIG. 25, a first mirror 2502and a second mirror 2504 are situated along an axis 2501 in aFabry-Perot configuration. The mirrors 2502, 2504 are secured to atemperature stable rod 2506 that defines a reference length. The rod2506 can have a square, circular, rectangular, or other cross section.In still other examples, reference path differences can be provided inan optical fiber or in other bulk dielectric media or waveguides. Insome cases, waveguides or bulk media are situated in hermeticallysealed, selected, pressure resistant containers to avoid lengthperturbations due to environmental causes. Connections to suchwaveguides or bulk media can be made with optical fibers, if desired.

Dynamic Frequency Selection

Laser radars with dual lasers have distinct advantages when Dopplereffects on measurements are of concern. Such so-called “measurementDoppler” is caused by movement of laser radar optics with respect to thetarget being measured. Several dual laser approaches include (1) thesuperposition of two completely separate systems (LO, reference arm,etc), (2) a system where the two lasers are separated by polarization,and (3) a system where the two lasers are separated by their LOfrequency. The third approach using LO frequency separation is typicallya lowest cost option but has additional constraint that the two LOfrequencies must be picked to keep the measurements separated for signalprocessing reasons by say by 1 MHz. For systems using fixed LOfrequencies this leads to lower performance in the Laser Radar.Disclosed herein are systems and methods in which laser chirp rates canbe adjusted depending upon a target distance.

As shown in Rezk et al., U.S. Patent Application Publication2011/0205523, in a dual laser system with a first laser chirping up at afirst rate and a second laser chirping down as a second rate, a rangemeasurement can be found based on both the first and second chirps as:

${{M\; 1} = {\frac{f_{1{up}} + f_{2{down}}}{2} = {\frac{f_{1} + f_{d} + f_{2} - f_{d}}{2} = \frac{f_{1} + f_{2}}{2}}}},$wherein f₁ and f₂ are independent ranges estimate, and f_(d) is aDoppler contribution to the heterodyne frequency. Range errors can bereduced by using large heterodyne frequencies. However, heterodynefrequency should also be kept within practical detection bandwidths.Typically, in dual laser systems, range error and noise effects areassociated with the lower heterodyne frequency.

Laser chirp rates can be selected based on target range. For each laserof a dual laser system, range is estimated based on an associated scalefactor R_(L) which can be conveniently expressed in units of MHz/m. Asan example, a system that provides variable or dynamic chirp rates canbe based on a laser having a coherence length of 30 m (maximum range), amaximum heterodyne frequency bandwidth of 60 MHz, a minimum frequencyseparation of 1 MHz, and a minimum target distance of 1 m. With fixedchirps, heterodyne frequencies for a target distance of 1 m are shown inthe following table.

Dual Laser Radar Fixed Chirp Rates Total Range R_(L1) f₁ R_(L2) f₂ 30 m2 MHz/m 60 MHz 1 MHz/m 30 MHz  1 m 2 MHz/m  2 MHz 1 MHz/m  1 MHz

However, in a representative dynamic system, chirp rates for one or bothlasers can be varied. For example, in the following table, the chirprate associated with the second laser is varied.

Dual Laser Radar Variable Chirp Rates Range R_(L1) f₁ R_(L2) f₂ 30 m 2MHz/m 60 MHz 1.966 MHz/m 59 MHz  1 m 2 MHz/m  2 MHz    1 MHz/m  1 MHzVariable chirp rates tend to improve noise performance and providesuperior Doppler correction at longer ranges as the two heterodynefrequency magnitudes are closer together. Typically, calibration andchirp linearization are used as well, based on, in part, measurement ofa reference length such as those described above.

A representative dual laser radar system 2600 in which laser chirp ratesare variable is illustrated in FIG. 26. A first laser driver 2602 iscoupled to a first laser source 2606 so as to produce a first beam thatis provided to a fiber coupler 2612. A second laser driver 2604 iscoupled to a second laser source 2608 so as to produce a second beamthat is combined with the first beam in the fiber coupler 2612. Thecombined beams are directed to a measurement/LO optical path 2614 and areference optical path 2616. Reference length heterodyne frequencies aregenerated at a calibration detector 2618 based on the first and secondbeams as combined with portions that propagate along a reference path.The calibration detector 2618 is coupled to a signal processor 2620 thatcan determine suitable scale factors for one or both of the first andsecond laser beams.

Measurement beams from a target and LO beams are returned from themeasurement/LO optical path 2614 and coupled to a detector 2622 so as toproduce first and second heterodyne frequencies associated with targetrange for the first laser beam and the second laser beam, respectively.The first and second heterodyne frequencies are coupled to the signalprocessor 2620 that provides a range estimate based on the heterodynefrequencies and the scale factor. A range selector 2624 is coupled to alaser source frequency controller 2628 so as to provide suitable laserdrive control signals or control data to the laser drivers 2602, 2604.The range selector 2624 is configured to select a chirp rate so as toobtain a heterodyne frequency in a predetermined range or at apredetermined value for one or both to of the first laser source 2606and the second laser source 2608. For example, if the first heterodynefrequency is larger or smaller than preferred, the chirp rate of thefirst laser source can be decreased or increased, respectively. Therange selector 2624 can be arranged so that a heterodyne frequency for aparticular feature of interest (on all features of interest) issubstantially constant by varying laser source chirp rate. For example,if a measured first heterodyne frequency is 1 MHz at a scale factorR_(L), the chirp rate and a first scale factor R_(L1) can be increasedby a factor of 50 so as to produce a 50 MHz heterodyne frequency. Chirprates may be limited due to laser source characteristics, but withinsuch bounds the laser source chirp can be changed as convenient.

FIG. 27 illustrates a representative laser range finding method. At2702, laser chirp rates are selected for one or more laser sources. At2704, the chirp rates and laser frequency sweeps are calibrated todetermine scale factors and deviations from an intended chirp profile.Typically, linear chirp profiles are selected and deviations fromlinearity can be measured with a reference length. Deviations fromlinearity and scale factors are stored for use in providing rangeestimates. Stepped, polynomial, exponential or other chirp rates can beused, although such chirp profiles are typically more difficult toestablish. At 2706, target range is estimated using the selected chirprates, and at 2708 preferred chirp rates can be selected for some or alllasers based on the measured target range. Chirp rates can be selectedso that heterodyne frequencies are within a preferred range, or aregreater than a preferred minimum or less than a preferred maximum value.If one or more chirp rates are selected that are different than currentchirp rates as determined at 2710, frequency sweeps are calibrated againat 2704 and additional target distance measurements are obtained.

In the above examples, chirp rates for multiple lasers are changed, butchirp rate can be changed in single laser laser radar systems as well.

Penta-Mirror Scanning

Measurement rates in laser radar systems can be limited by the rate atwhich a measurement beam can be scanned over a target. In someconventional systems, relatively massive optical systems and componentsmust be rotated so that high speed scanning is difficult and expensive.Representative scanning systems and methods described below can addressthese and other limitations of conventional approaches.

With reference to FIG. 28, a laser radar system 2800 includes at leastone chirped laser 2802 that is coupled to a fiber coupler 2804. Thefiber coupler delivers portions of a chirped laser beam to a localoscillator beam optical system 2806 and a measurement beam opticalsystem 2808 that produce an LO beam and a measurement beam,respectively. As shown in FIG. 28, the LO beam optical system 2806 isconfigured to couple the LO beam through a focus adjustment corner cube2810 for reflection by an LO corner cube 2812 and an LO return reflector2814, preferably implemented as a retroreflector. The measurement beampickup/delivery optical system 2808 directs a diverging measurement beam2816 to the focus adjustment corner cube 2810 and to a return reflector2818. In this configuration, the LO is a “remote LO” in that pathdifferences associated with the focus adjustment corner cube 2810 aresubstantially the same for the measurement optical path and the LOoptical path, i.e., four passes through the focus adjustment corner cube2810 for the LO beam and the measurement/return beams.

The laser radar system 2800 is configured so that the measurement beamis directed along an axis 2820 to an objective lens 2822 to anelevational scan assembly 2824. A return beam is collected by theobjective lens 2822 and coupled to the fiber coupler 2804 along areverse of the measurement beam optical path. A receiver 2830 is coupledto receive the combined beams and produce a signal at a heterodynefrequency that is coupled to a system controller 2834 configured toprovide range estimates based on heterodyne frequencies.

The system controller 2834 can include or be based on a personalcomputer or other computing device (not shown in FIG. 28) such as alaptop, tablet, a workstation, or a handheld communication device. Thereceiver 2830 is configured to produce a recognition signal for thepersonal computer, and the personal computer can calculate or compute arange estimate based on detected heterodyne frequencies. In someexamples, the system controller 2834 can include one or more computersthat can be in a common location or coupled via a wired or wirelessnetwork such as a local area network or a wide area network. A firstcomputer can receive signals from the receiver 2830 and forward thereceived signals or digital or other representations thereof to a secondcomputer using a wired or wireless communication network orcommunication link. The second computer establishes range estimatesusing received signals based on heterodyne frequencies.

The elevational scan assembly 2824 includes bearings 2840 configured forrotation about the axis 2820. The bearings 2840 typically includeencoders as well that permit determination of rotational angle. Firstand second reflectors 2842, 2844 are situated to direct the measurementbeam along a rotatable axis 2850.

The elevational scan assembly 2824 and the LO and measurement beamoptical systems can be secured to a base 2851 that is coupled to ansecond scanner 2852 that is configured to rotate the base 2850 about anaxis 2858. Scanning of the measurement beam is directed by the controlsystem 2834 that is coupled to the elevational scan assembly 2824 andthe secondary scanner 2852. The control system 2834 is also coupled to atranslation stage 2853 that is situated to translate the focusadjustment corner cube 2810 in a direction parallel to the axis 2820 soas to focus the measurement beam at a target surface.

A camera 2860 can also be provided for viewing a target area. The camera2860 can be situated to image along the axis 2850 through the reflector2844. In representative examples, the measurement beam is an infrared ornear-infrared beam and the reflector 2844 can be configured to transmita visible beam and reflect the measurement beam. For example, thereflector 2844 can be a so-called “hot mirror” that reflects infraredradiation and transmits visible radiation. In other examples, the camera2860 is situated to image along an axis 2868 that is displaced from andpossibly tilted with respect to the measurement axis 2850. The camera2860 is generally fixed with respect to or secured to the elevationalscan assembly 2834 so that a visible image of a target can be obtainedor monitored during scanning. In addition, since the camera 2860 moveswith and is aligned to the measurement beam, the camera output image canbe used in a variety of ways to provide additional metrologyinformation.

FIGS. 29A-29C illustrate alternative elevational scan assemblies forlaser radars. As shown in FIG. 29A, measurement and return beams 2904are directed along an axis 2906. An elevational scan assembly 2910includes a single reflector 2912 that can have a wavelength dependentreflectance to permit imaging of a target area with a camera 2914. Theelevational scan assembly is configured to rotate about the axis 2916(an extension of the axis 2906). FIG. 29B illustrates a representativescan assembly 2930 that includes bearings 2932 configured to providerotation about an axis 2934. A pentaprism 2936 is configured to directthe measurement beam to a target (and a return beam to a detectionsystem). Pentaprism surfaces can be coated or total internal reflectioncan be used. If total internal reflection is used, polarizationcompensation may be required if the laser radar is polarizationsensitive. A wedge prism 2938 can be secured to the pentaprism 2936 topermit a camera 2940 to view a target area. If such a wedge prism isused, a wavelength dependent dielectric coating 2942 is provided toreflect the measurement and return beams to and from the target, andtransmit a viewing beam to the camera 2940. FIG. 29C illustrates apentaprism 2946 and a wedge prism 2948 configured to transmit a viewingbeam to a camera via a different pentaprism face. It will be apparentthat other examples are possible.

A representative laser radar optical assembly that includes a rotatablepentaprism scanner is illustrated in FIGS. 30A-30B. A beam shaping andbeam collection optical system 3002 includes a corner cube 3004 and areturn reflector 3006 that are situated along an axis 3008 with anobjective lens 3010 so as to produce a focused measurement beam. Apentaprism 3012 is configured to reflect the measurement beam from theoptical system 3002 at surfaces 3014, 3016 so as to direct themeasurement beam to the target. While a pentaprism 3012 is convenient,mirrors can be arranged so as to produce similar reflections. Thepentaprism 3012 is secured to an elevational rotational stage 3018 thatis rotatable about an elevational axis 3020. Elevation bearings 3019A,3019B permit rotation about the axis 3020 so that the measurement beamis directed along a rotatable axis 3022. In some examples, only one ofthe elevation bearings 3019A, 3019B is used.

A reference mirror 3026 is secured to a base 3028. The axis 3022 can berotated so that the measurement beam can be directed to the referencemirror 3026 through an aperture 3027. The reference mirror 3026 can beused to establish a reference length for calibration, and pathdifferences in an LO beam can be compensated based on the calibration.Elevational angles can be detected with one or more encoders such asencoders 3021A, 3021B, rotations about an azimuthal axis can be providedwith an azimuthal rotational stage 3047 and the base 3028 can be rotatedabout an axis 3030.

FIG. 30B is similar to FIG. 30A but is a top view with the pentaprism3012 rotated 90 degrees about the axis 3020. A camera 3040 includes alens 3042 that is situated so as to image in a field of view 3038 alongan axis that is parallel to and displaced from the axis 3022. One ormore azimuth encoders such as encoder 3044 permits determination ofazimuthal rotations produced with the azimuthal stage 3047.

In some examples, the axis 3022 as folded corresponds to the elevationalaxis 3020. The pentaprism 3012 tends to reduce beam pointing errorsassociated with elevational axis bearing wobbles, and provides increasedoptical path length. In FIGS. 30A-30B, laser sources, local oscillatoroptics, and control and processing systems are not shown for clarity. Anoptical fiber can be used to deliver a chirped beam to the opticalsystem 3002 as well as couple returned measurement beam portions and anLO beam to a detection system as shown in the examples above.

If the reference mirror 3026 is a curved mirror, azimuth bearing wobblecan be detected and estimated by directing the measurement beam to acenter of curvature of the reference mirror 3026. Referring to FIGS.30C-30D, a curved reference mirror 3060 is provided, and apparatus ofFIGS. 30A-30B is arranged so that the measurement beam is directed to acenter of curvature 3061 of the reference mirror 3060. An xyz coordinatesystem 3062 is also shown. Rotational errors about the x-axis (such asazimuthal wobble) cause return signal amplitude to decrease, and thissignal loss can be recovered by adjusting the azimuthal angle, i.e.,with a corresponding scanner rotation about the x-axis. Wobble in otherdirections also causes signal loss, but such signal loss is notgenerally recoverable with an elevational adjustments. For example, asshown in FIG. 30D, a rotational error θ is not correctable with arotation about the x-axis.

FIG. 49 illustrates an optical system in which an LO retroreflector 4902is secured to or situated at a surface of a scannable pentaprism 4904. Acorner cube 4908 and a return reflector 4906 are situated formeasurement beam focusing with a lens 4912, and an LO reflector 4910 issituated to reflect an LO beam into the corner cube 4908. Thisconfiguration permits movement of the LO corner cube 4902 away from apropagation axis of the measurement beam. Additional details are similarto those of other examples, and one or more LO beams can be providedwith similarly situated LO corner cubes.

The elevational scan assembly in above embodiment is not limited to tworeflective surfaces situated at an angle of 45 degrees, but can also beconfigured so that the two reflective surfaces are at angles of 10, 20,30, 40, 50, 60, 70, 80, 90, 100, 120, 130, 135, 140, 150, 160, or 170degrees. The elevational scan assembly may be situated at anintersection of an axis associated with beam propagation to theelevational scan assembly from the laser (i.e., a beam delivery axis),and a target direction from the elevational scan assembly. Duringrotations of the elevational scan assembly, an angle between the beamdelivery axis and the target direction from the elevational scanassembly may be constant or approximately constant. For example, thisangle may be constant or approximately constant during from 10% to 100%of a predetermined period of rotation. In addition, in other examples,positions of the elevational scan assembly and the pentaprism 2936 canbe measured in other ways. For example, the position of the pentaprism2936 can be measured with an interferometer so that the interferometershows position changes during rotation. The result may be used tocorrect or compensate target distances estimates.

In other examples, the elevational scan assembly can be movable alongthe axis 2850 or other axes. Such movement can be used to select aposition of the measurement beam on at least one of the reflectivesurfaces.

Fiber Based Laser Radar Systems

Referring to FIG. 31, chirped (typically counterchirped) laser beams atfirst and second wavelengths λ1, λ2, respectively can be coupled to beamshaping, scanning, and collection optics using an optical fiber system3100. First and second chirped laser beams from first and second lasersare coupled to optical fibers that are in turn coupled to respectivefiber couplers 3102, 3104. The fiber couplers 3102, 3104 are secured toa laser radar optical support 3101 that is situated so as to rotate withat least one of an elevational or azimuthal rotation stage. The fibercouplers 3102, 3104 include two outputs 3102A, 3102B and 3104A, 3104B,respectively. A portion of the first chirped laser beam is coupled tothe output 3102A to serve as a first LO beam, while another portion isdelivered to a third fiber coupler 3110 from the output 3102B.Similarly, a portion of the second chirped laser beam is coupled to theoutput 3104A to serve as a second LO beam, while another portion isdelivered to the third fiber coupler 3110. The portions of the first andsecond chirped laser beams from the outputs 3102B, 3104B are combined inan output 3112 of the third fiber coupler 3110 to produce a combinedbeam (with λ₁ and λ₂ beams) that can be used a measurement beam. Thecouplers shown in FIG. 31 typically are 2 by 2 couplers and haveadditional input or output ports, but unused ports are not shown. Suchunused ports are usually terminated to reduce back reflections. Thecouplers and optical fibers are preferably polarization retaining, butother fibers and couplers can be used. Coupler split ratios aretypically selected so that an input beam is divided into twosubstantially equal portions, but other split ratios can be used.Measurement beam and LO beam optics of various kinds can be used, butare not shown. In this configuration, the measurement beam paths foreach of the first and second chirped laser beams are the same.

Referring to FIG. 32, a fiber based, dual wavelength, chirped laserradar system includes a first laser 3202 and a second laser 3204 thatare optically connected to a 2 by 2 coupler 3206. Combined first andsecond laser beams are available at a coupler output 3206A (and a secondoutput, not shown). The coupler output 3206A is connected to a 2 by 2coupler 3208. A coupler output 3208A is connected to an opticalcirculator 3210 so as to communicate an optical beam from a circularport 3210A to a circulator port 3210B. A combined beam associated withlaser beams from the first and second lasers 3202, 3204 is available atthe circulator port 3210B. A lens 3214 and a quarter wave retarder 3216are situated to direct the combined beam as an LO beam along an LO pathin a circular SOP to a reflector 3218. The return beam is directed tothe circulator 3210 and to port 3210C of the circulator. Circulator port3210C is connected to a port 3224A of a 2 by 2 coupler 3224.

The coupler output 3208B is connected to an optical circulator 3230 soas to communicate an optical beam from a circulator port 3230A to acirculator port 3230B. A combined beam associated with laser beams fromthe first and second lasers 3202, 3204 is then available at thecirculator port 3230B. A port 3240A of a dual band coupler 3240 isconnected to the circulator port 3230B and to a viewing or pointinglaser 3244. Combined first and second chirped laser beams and a viewinglaser beam are then available at a coupler port 3240A. A lens 3250 and aquarter wave retarder 3252 are situated to direct the combined beams asmeasurement and viewing beam to a target, and receive a return beamassociated with the first and second chirped lasers from the target. Thereturn beam is directed to through the dual beam coupler 3240 to thecirculator 3230 and to a port 3224B of the 2 by 2 coupler 3224. The 2 by2 coupler 3224 thus receives the return beam at port 3224A and the LObeam at 3224B and delivers a mixture to a port 3224C that is coupled toa detection system configured to detect heterodyne frequenciesassociated with one or both of the chirped lasers and provide a rangeestimate. In this example, the detection system can be located remotelyfrom movable (scanned) optics, but the return beam and the LO beam bothcontain contributions from each of the two chirped lasers. The combinedbeams can be brought to the scanning optics from the coupler 3206 on asingle optical fiber to reduce the number of fibers needed.

Referring to FIG. 46, a fiber system for producing LO beams and a duallaser measurement beam includes first and second laser sources 4602,4604 coupled to respective 2 by 2 beam splitting couplers 4606, 4608.These couplers typically provide approximately even power splitting tocoupler outputs, but other split ratios can be used. One output of eachof the couplers 4606, 4608 is coupled to 2 by 2 beam combining couplers4610, 4612, respectively. The remaining outputs of the couplers 4606,4608 are connected to a laser combining 2 by 2 coupler 4614 which isconfigured to couple a dual wavelength measurement beam to a circulator4616. The circulator 4616 couples the dual wavelength measurement beamto a polarization maintaining single mode fiber 4618 which can providethe measurement beam to laser radar scanning optics.

A return beam from a target is coupled by the circulator 4616 to areturn beam splitting 2 by 2 coupler 4620 that directs portions of thereturn beam to the beam combining couplers 4610, 4612. PIN diodedetector 4624 is configured to receive a portion of the return beam andthe first LO beam from the coupler 4610 and produce a heterodynefrequency associated with the chirp of the laser 4602. PIN diodedetector 4626 is configured to receive a portion of the return beam andthe second LO beam from the coupler 4612 and produce a heterodynefrequency associated with the chirp of the laser 4604. In this example,the LO beams are coupled to the return beam via optical fiber, and arenot directed to measurement beam scanning or focusing optics.

In another example shown in FIG. 47, a fiber system for producing LObeams and a dual laser measurement beam includes first and second lasersources 4702, 4704 coupled to respective 2 by 2 beam splitting couplers4706, 4708. These couplers typically provide approximately even powersplitting to coupler outputs, but other split ratios can be used. Oneoutput of each of the couplers 4706, 4708 is coupled to respective LOoptical circulators 4707, 4709. These circulators couple the LO beams tolaser radar focusing and scanning optics. The remaining outputs of thecouplers 4706, 4708 are connected to a laser combining 2 by 2 coupler4714 which is configured to couple a dual wavelength measurement beam toa circulator 4716. The circulator 4716 couples the dual wavelengthmeasurement beam to laser radar focusing and scanning optics.

LO beams returned from the laser radar focusing and scanning optics aredirected by the LO optical circulators 4707, 4709 to the beam combining2 by 2 couplers 4710, 4712, respectively. A return beam from a target iscoupled by the circulator 4716 to a return beam splitting 2 by 2 coupler4720 that directs portions of the return beam to the beam combiningcouplers 4710, 4712. PIN diode detector 4724 is configured to receive aportion of the return beam and the first LO beam from the coupler 4710and produce a heterodyne frequency associated with the chirp of thelaser 4702. PIN diode detector 4726 is configured to receive a portionof the return beam and the second LO beam from the coupler 4712 andproduce a heterodyne frequency associated with the chirp of the laser4704. In this example, the LO beams are coupled in part to measurementbeam scanning and focusing optics to compensate LO/measurement beamoptical path differences.

The fiber combiner/splitter of FIG. 47 can be coupled to laser radarfocusing and scanning optics with the arrangement shown in FIGS.48A-48B. An input beam support 4802 is configured to receive and retainan optical fiber that communicates a dual laser measurement beam in anaperture 4804. The beam aperture 4804 can be defined by support arms4805 that extend into a larger aperture 4807 configured to transmit ameasurement beam. Collimators 4808, 4810 are configured to receive LObeams associated with respective lasers. LO beams are directed by thecollimators 4808, 4810 to a focus adjustment corner cube 4812 and an LOcorner cube 4814. The LO corner cube 4814 returns the LO beams to thefocus adjustment corner cube 4812 and to respective retroreflectors4816, 4818. Other optical elements that shift one or more of the LObeams and returns the shifted LO beam or beams to the focus adjustmentcorner cube 4812 can be used. From the retroreflectors 4816, 4818 the LObeams follow a reverse path back to respective collimators for couplingto corresponding detectors. A representative LO path 4817 isillustrated. The measurement beam is directed to the focus adjustmentcorner cube 4812 and a return reflector 4828 and then to a lens 4820that focuses the measurement beam at a target. A representativemeasurement beam path 4819 is shown in FIG. 48A.

Beam Steering Error Detection and Correction

FIG. 33 illustrates a fiber coupled laser tracker or laser radar system3300 in which an optical flux from a fiber 3302 is directed to a cornercube 3304. A partially transmissive return reflector 3306 returns aportion of the optical flux through the corner cube 3304 to a lens 3308that forms a focused interrogation beam that is directed to a target3310. The other portion of the optical flux is coupled to a positiondetection lens 3311 that directs the transmitted flux to a positiondetector 3312 such as a quadrant detector, a detector array, or otherdetectors so that beam location can be estimated. In this way, alocation of an output surface 3301 of the fiber 3302 can be estimated.For convenience, the corner cube 3304 is illustrated as a right angleprism. Portions of the interrogation beam that are returned to the lens3308 from the target are directed to the return reflector 3306. Thereturn reflector 3306 is partially transmissive so that some of thereturned interrogation beam is also coupled to a position detection lens3311 that directs the transmitted flux to a position detector 3312 butthe magnitude of this flux is typically too low to generate a usefulsignal.

The detector 3312 is coupled to a tracking processor 3320 thatdetermines fiber position based on electrical signals from the detector3312. Based on the estimated fiber position, an estimated beam positioncan be determined and a beam position controller 3324 can direct beamadjustment. Alternatively, an estimated beam position can be used incorrecting position information in processing returned optical flux toestablish object surface profiles, distances, or other objectproperties.

The reflector 3306 is typically configured to transmit less than about10%, 5%, 1%, or 0.5% of an incident flux. In the configuration of FIG.33, the return reflector is part of a focusing system, so that couplingflux to a detector for beam tracking does not otherwise disturb anoptical system, except for a change in transmittance. However, in otherexamples, beam portions transmitted by fold mirrors can also be used fortracking. In addition, in some cases, some beam portions escape thecorner cube upon each reflection at rear surfaces, and these beamportions can also be directed to a detector for beam tracking. If atracking error or displacement or tilt of an optical module or componentis detected, a compensation or calibration procedure can be executed.

FIG. 34 illustrates a laser radar system that includes an optical module3404 coupled to receive an input optical beam (such as a chirped laserbeam) from an optical fiber 3406. The optical module 3404 includes abeam splitter 3408, a retroflector 3410, and a detector 3412. Lenses3416-3418 are provided to shape and focus optical beams as needed. Areflector 3420 is situated to direct an optical beam from the opticalmodule 3404 to a focus adjustment corner cube 3424. A partiallyreflective return mirror 3430 is situated to receive an optical beamfrom the focus adjustment corner cube 3424 and reflect a measurementbeam to the focus adjustment corner cube 3424 and an objective lens3434. The focus adjustment corner cube 3424 is translatable along anaxis 3438 so as to focus the measurement beam at a target surface.Scattering, reflected, or other portions of the measurement beam fromthe target are directed back to the optical module 3404 and coupled tothe detector 3412. A portion of the input optical beam from the opticalfiber 3406 is reflected to the detector 3412 by the beam splitter 3408as an LO beam.

The optical module 3404 can be arranged so that an input beam in alinear SOP (for example, horizontal or “H”) is slightly reflected by thebeam splitter 3408 to the detector 3412 to provide an LO beam, while theremainder of the input beam is transmitted. A quarter wave plate 3419produces a first circular SOP from the H SOP. The return beam from thetarget is preferentially a second circular SOP in a handedness oppositethat of the first SOP. The quarter wave plate 3419 produces a vertical(V) polarization that is reflected by the beam splitter 3408 to aquarter wave plate 3421 and the retroreflector 3410. The quarter waveplate 3421 then produces a first circular SOP that is reflected by theretroreflector 3410 as a second circular polarization which is convertedinto an H polarization by retransmission by the quarter wave plate 3421.The return beam from the target is thus converted into an H SOP whichcan be efficiently transmitted by the beam splitter 3408 to the detector3412. Thus, the optical module serves to provide separate measurementand LO beams from a common input beam, and recombine the return beam andthe LO beam at a detector.

As shown in FIG. 34, a reflector 3420 is situated to direct the inputbeam to the target, but the optical module 3404 can generally besufficiently compact that such a reflector is not required, and the beamfrom the lens directed into the corner cube 3424 without reflection.

The partially reflective return mirror 3430 also transmits a portion ofthe measurement beam to a lens 3450 and a position detector 3452.Measurement beam shape, pointing direction, and other characteristicscan be evaluated based on the beam as imaged at the position detector3452. For example, beam pointing errors can be evaluated, or variationsin optical component locations can be detected. Any errors or artifactsdetermined in this manner can be used to adjust beam pointing or opticalelement position and orientation, or to provide compensation data sothat scan errors can be corrected in the presence of beam positionerrors. An error processor 3454 can be provided to determinecompensation or correction values, or to report the presence of errors.

Partial Remote Local Oscillators

With reference to FIG. 35, an optical fiber 3502 is situated to directan input beam to a corner cube 3504 that is translatable along an axis3506 so that a measurement beam 3509 can be focused at a target with alens 3508. The input beam is incident on a return mirror 3510 thatreflects a measurement beam portion of the input beam to the lens 3508.An LO beam portion is transmitted to form an LO beam 3520 that isfocused at a detector 3516 with an outer portion 3512A of a lens 3512. Aportion of the return beam from the target is transmitted by the returnmirror 3510 and focused by an inner portion 3512B of the lens 3512 as abeam 3518. The outer portion 3512A and the inner portion 3512B of thelens have different curvatures so that the LO beam and the return beamare suitably focused at the detector 3516. A signal processor 3524 isconfigured to estimate target range using a heterodyne frequencyproduced by the combined beams. The lens 3512 can be formed as a singlelens element with differing curvatures, or be assembled from one or moreseparate lenses. In order to keep beams focused on the detector 3516,the lens 3512 may be secured to a translation stage so as to betranslated in conjunction with the motion of the corner cube 3504. Inthe configuration of FIG. 35, an LO beam is provided by transmissionthrough the return reflector and the LO beam traverses the focusadjustment corner cube 3504 once, and has a common optical path with themeasurement beam to the return reflector 3510. The measurement beam andthe associated return beam make a total of four passes through the focusadjustment corner cube 3504. As a result, path differences between themeasurement/return beam and the LO beam introduced by focusing opticstend to be less well compensated than in systems in which the LO beamand the measurement/return beam make the same number of transits.

Fiber Beam Delivery with Compact Bulk Optics

The optical module 3404 shown in FIG. 34 can be used in variousembodiments to separate and recombine measurement and LO beams. As shownin FIG. 36, an optical module 3602 is secured to an azimuth/elevationalangle scanner 3604 that is controlled to scan a measurement beam withrespect to a target 3608. A lens 3610 is situated to receive themeasurement beam and direct the measurement beam to the target 3608.Fiber coupled laser diode sources 3612, 3614 are connected to inputports of a 2 by 2 fiber coupler 3618. An output port 3618A of the fibercoupler 3618 receives portion of input beams from each of the sources3612, 3614 and an optical fiber 3620 couples the combined beams to theoptical module 3602. The module 3602 can produce an LO beam and ameasurement beam, and combine the return beam and an LO beam at adetector or couple the combined return/LO beams to a fiber for couplingto a detector that may or may not be fixed with respect to the stage3604. Details are not shown in FIG. 36 but can be similar to those ofFIG. 34.

The coupler 3618 also includes an output port 3618B that is connected toa reference length coupler 3630 configured to direct a portion of thecombined beam to a reference length 3632 via an output port 3630A. Thereference length 3632 is configured to reflect the combined beam back tothe output port 3630A but delayed by a reference distance. An outputport 3630B of the fiber coupler 3630 is terminated to reflect a portionof the combined beam to provide an LO beam. The beam from the referencelength 3632 and the LO beam are coupled to an optical fiber 3634 and areference detector 3636.

With reference to FIG. 37, an optical module 3702 is situated to receiveone or more input laser beams such as one or more chirped laser beamsfrom an optical fiber 3703. A portion of the received beams is coupledto a detector 3704 as an LO beam and another portion is directed to alens 3706 as a measurement beam. The lens 3706 focuses the measurementbeam at a target 3707 and couples a return beam to the optical module3702 for delivery to the detector 3704. In the example of FIG. 37,measurement beam focusing can be provided by translation of the opticalmodule 3702 and/or the lens 3706.

In another example, shown in FIG. 38, a focus adjustment corner cube3802 and a return reflector 3804 are situated so that translation of thefocus adjustment corner cube 3802 focuses a measurement beam at atarget. An optical fiber 3810 couples one or more input laser beams toan optical module 3806 that couples a combined measurement beam to thefocus adjustment corner cube 3802. A lens 3812 is situated to receivethe combined measurement beam and direct the combined measurement beamto a target as well as couple a returned beam to the optical module3802.

FIGS. 39-40 illustrate addition embodiments similar to those of FIGS.37-38, but in these examples folding mirrors 3902, 4002 are secured totransducer/scanners 3904, 4004, respectively. The scanners 3902, 4002can be configured to scan a measurement beam based on rotations aboutone or more axes.

With reference to FIG. 50, an LO beam separating and measurement beamcombining optical system includes prisms 5004, 5006, 5008 that definereflective surfaces 5014, 5016, 5020. The reflective surface 5016 can beprovided with different reflective coatings as surface portions 5016A,5016B. A first laser is coupled to a first collimating lens 5022 and apolarizer 5026 along an axis that is offset from an axis of the firstlens 5022. The first lens 5022 and the polarizer 5026 can be secured tothe prism 5004. The polarizer is configured to transmit an x-directedlinear SOP but is tilted so that a smaller portion (typically 1-10%) ofa y-polarization is transmitted as well. Such a polarizer axis 5027 isshown as viewed along the axis of the first lens 5022. The reflectivesurface 5014 is selected so as to reflect a y-polarization and transmitan x-polarization. Thus, a portion of the first laser beam is directedalong a remote local oscillator (RLO) path in linear SOP that isperpendicular to the plane of FIG. 50. The reflective surface portion5016A is polarization independent and is selected to transmit andreflect equally (though unequal values can be used). As a result, 50% ofthe first laser beam is directed along a light loss path and 50%directed along the measurement path, polarized in the plane of FIG. 50.

A second laser is coupled to a second collimating lens 5024 along anaxis of the lens 5022. The second laser beam is input as anx-polarization. The reflective surface portion 5016B is configured aspolarization independent reflective surface that reflects a portion(typically about 20%) of the second laser beam towards the RLO path andtransmits another portion (typically about 80%) to the reflectivesurface 5020. Because the reflected portion is linearly polarized in theplane of FIG. 50, the reflective surface 5014 transmits substantiallyall of this portion to the RLO path. First and second LO beams are thusgenerated, one in an SOP perpendicular to the plane of the drawing (thefirst laser beam) and the other in an SOP in the plane of the drawing.In addition, the first and second LO beams are not parallel to eachother due the offset of the first laser beam with respect to the firstlens 5022.

The transmitted portion of the second laser beam is reflected to thereflective surface portion 5016B that reflects portions (typically equalportions) to the measurement path and the light loss path. An angle ofthe reflective surface 5020 is selected so that the first and secondmeasurement beams propagate along parallel axes upon exiting a lens5030. Parallel propagation can be obtained by selection of surfaceorientation for the reflective surface 5020 or other surfaces, bytranslations of one or more collimating lenses or translations of one ormore fibers that deliver the first and second laser beams.

The LO beams and return beam can be recombined for heterodyne frequencydetection. Because the LO beams propagate along different axes anddifferent SOPs, these beams can be selectively coupled to correspondingdetectors, and each LO beam can be coupled to only one detector withlittle leakage to the other. In some examples, 60 dB or more LOisolation can be provided.

Multiple Beam Scanning and Vision Systems

Referring to FIG. 41, a laser radar that scans a plurality ofmeasurement beams includes first and second chirped lasers 4102, 4104that are connected to optical isolators and a 2 by 2 fiber coupler 4108with polarization maintaining fibers. The beams from the first andsecond lasers are configured to be aligned with either a fiber fast axisor a fiber slow axis. The outputs of the fiber coupler are situated sothat lenses 4112, 4114 produce collimated beams that are directed to abeam dividing optical system 4120 that includes a plurality of beamsplitters and reflectors (shown as prisms in FIG. 41). Each of thelenses 4112, 4114 produces a beam that includes up and down chirpedportions and the beam dividing optical system 4120 is configured toproduce eight beams that are directed to a polarizing beam splitter(PBS) 4124. The beams and the PBS 4124 are arranged so that a portion(typically about 5%) of each beam is reflected by the PBS 4124 at areflective surface 4125 to a corresponding detector of an array 4128 ofdetectors to serve as LO beams (with LO portions for both up anddownchirped lasers).

Eight beam portions are transmitted by the PBS 4124 to a quarter waveplate 4130 and a prism or lens array 4134 that directs eight beams to ascanner 4140. The eight beams are then scanned over a target area, andreturn beam portions from the target are directed to respectivedetectors of the detector array 4128. The lens array 4134 and thequarter wave plate 4130 direct the return beams to the reflectivesurface 4125 which is situated to reflect the return beams to a quarterwave plate 4144 and to respective retroreflectors 4151-4158 of aretroreflector array 4148. A single scanner such as a scanning mirrorcan be used, but in other examples, one or more or all beams can bedirected to associated scanners.

The quarter wave plate 4130 is arranged so that the measurements beamsare circularly polarized in a first circular SOP. The return beams arecircularly polarized in second circular SOP (orthogonal to the firstcircular SOP). Transmission of the return beams by the quarter waveplate 4130 produces linearly polarized beams that are reflected by thePBS 4114 to the quarter wave plate 4144 and the retroreflector array4148. As a result the beams arrive at the quarter wave plate 4130 in afirst linear SOP but are returned to the PBS 4114 in an SOP that istransmitted by the reflective surface 4125 to the detector array 4128.

In the example of FIG. 41, two lasers (one upchirped and the otherdownchirped) are provided so that Doppler effects can be correctedwithout multiple scans by each beam. If a lower overall scan rate isacceptable, a single chirped laser can be used and measurements obtainedduring its upchirp and downchirp combined. Simultaneous scans (withupchirped and downchirped lasers) are generally preferable to sequentialscans (with a single up and downchirped laser) for moving or vibratingtargets.

In an illustrative example, eight beams (with up and down chirped laserbeam components) are scanned simultaneously, so that a scan rate of 192lines/sec requires a scanning mirror to oscillate at 12 Hz. Each groupof eight lines will be scanned in the forward direction in 1/24th of asecond (0.04167 seconds) and then scanned in the reverse or retracedirection in the same time. If 4000 measurements/second can be made,then 167 measurement points per line can be acquired in 1/24 s and a32,000 pixel frame can be acquired in one second.

A rotating scan mirror introduces Doppler effects into the scanned beam.If a beam center strikes a scan mirror along an axis of rotation, beamedges will experience equal and opposite Doppler frequency shifts thatproduce an edge to edge frequency difference F_(dd)=4ωd/λ, wherein co isscan mirror angular speed (rad/sec), d is beam diameter, and λ is beamwavelength. For a 15 degree scan in 1/24 sec, the maximum Dopplerfrequency difference is about 32.4 kHz. Range errors associated with thefrequency difference tend to be about ⅓ as large as those predictedbased on the maximum Doppler frequency difference. For dual laser (i.e.,counterchirped) laser systems, range errors can be even less.

The scanner 4140 is generally configured to provide a fast scan of themeasurement beams. A scanning lens expands and focuses the scannedbeams. The scan angle of each of the beams is reduced in proportion tothe beam expansion as provided by the optical invariant. The expanded,focused beams are then directed to a secondary scanner that isconfigured to scan the beams in a direction that is not parallel to thescan direction of the scanner 4140. The secondary scanner can becontinuous and periodic so as to produce zig-zag scan patterns, or canscan in stepped increments to produce a series of parallel multiple beamscans. Other scan patterns can be used as may be convenient.

The system of FIG. 41 generally includes a reference path as well. Eachof the lasers is connected to a fiber coupler such as a 2 by 2 couplerthat directs portions of each laser to the system shown in FIG. 41.Other portions are directed to a reference path. For example, a 2 by 2coupler can be configured to provide 95% of each beam to the multiplebeam scanner and 5% of each beam to a reference length. A referencesignal for each laser is produced at a corresponding reference detector.

A representative range processing receiver is illustrated in FIG. 42. Asignal detector 4202 is coupled to receive a return beam and an LO beam.A bandpass filter 4204 is configured to filter the detector signal andremove signal contributions outside a frequency range of interest. Ananalog-to-digital convertor (A/D) 4208 is configured to produce adigital representative of the filtered detector signal, and a digitalsignal processor 4210 identifies a heterodyne frequency associated withtarget range based on, for example, an FFT of the filtered detectorsignal. A reference arm signal based on a reference beam and a referenceLO beam is produced at a reference detector 4222. A bandpass filter 4224is configured to filter the reference detector signal and remove signalcontributions outside a frequency range of interest. Ananalog-to-digital convertor (A/D) 4228 is configured to produce adigital representative of the filtered reference detector signal, andthe digital signal processor 4210 identifies a heterodyne frequency.Each of the beams of FIG. 41 is similarly processed, and reference beamsfor each laser are also processed. Eight identified ranges and tworeference heterodyne frequencies are further processed to provide rangeestimates and calibration.

Laser Radar with Apertured Folding Mirror

Referring to FIG. 43, an optical module 4302 is coupled to receive aninput beam from an optical fiber 4304. A measurement beam is directed toan aperture 4306 in a fold mirror 4308. A focusing system 4314 thatincludes a focus adjustment corner cube 4312 and a return reflector 4324is situated to receive the measurement beam and return the measurementbeam to a reflective surface of the mirror 4308. The mirror 4308 thendirects the measurement beam to a lens 4318 that focuses the measurementbeam at a target. Beam focusing is provided by translation of the focusadjustment corner cube 4312 in a direction parallel to an axis 4320.

The optical module 4302 and the focusing system 4314 are secured to anazimuthal rotational table 4330 that is configured to rotate about anaxis 4332. The folding mirror 4308, the focusing lens 4318, and thevideo camera are coupled to an elevational rotational bearing that isconfigured to rotate about the axis 4320.

Remote Local Oscillators with Compact Bulk Optics

FIGS. 44A-44B illustrate an optical system that couples LO beams and acombined dual laser measurement beam to a focus adjustment corner cube.FIG. 44A illustrates input LO beams and measurement beam, and FIG. 44Billustrates mixing of a return beam with the LO beams. Single modeoptical fibers can be used to deliver the LO beams and the combinedmeasurement beams, and single mode or multimode optical fibers can beused to couple mixed LO and return beams to detectors, but such fibersare not shown in FIGS. 44A-44B.

A PBS cube 4402 is configured to receive first and second LO beams LO1,LO2 and a combined dual wavelength measurement beam (M1/M2) thatpreferably are in a first linear SOP that is transmitted by the PBS cube4402. A prism 4404 directs the first LO beam to a focus adjustmentcorner cube and a first LO return reflector so as to produce a firstremote LO beam. A right angle prism 4406 and a rectangular prism 4408are configured to direct the second LO beam to the focus adjustmentcorner cube and a second LO return reflector so as to produce a secondremote LO beam. The combined measurement beam is focused by a lens 4410and propagates to a mirror that reflects the beam to a focus adjustmentcorner cube assembly and then to a target. A quarter wave plate 4407 isorientated to produce a common circular SOP in the LO beams and themeasurement beam.

FIG. 44B illustrates first and second LO beams as returned to the PBScube 4402. The quarter wave plate 4407 produces a second linear SOP thatis reflected by the PBS cube 4402. The first LO beam is reflected to areflective surface 4422 defined as an interface surface betweenparallelogram prisms 4420, 4424 to a second output (OUT2). The returnmeasurement beam is reflected so as to be partially reflected at asurface 4426 and a surface 4423 so as to be coupled to both the firstand second outputs (OUT1, OUT2). The surface 4426 can be defined at aninterface between the parallelogram prism 4424 and a parallelogram prism4430. The second LO beam is reflected by the PBS cube 4402 so as to bereflected by a surface 4428 to the first output (OUT1). Typical surfacereflectances for the surfaces 4422, 4426, 4428 are 80%, 50%, and 20%,respectively, but other reflectances can be used. Additional prisms canbe provided so that the assembly of FIGS. 44A-44B can be formed as acemented optical assembly.

FIG. 45 illustrates an alternative to that of FIGS. 44A-44B. Instead ofdirecting first and second LO beams to a focus adjustment corner cube,reflectors 4502, 4504 are situated at a surface of a quarter wave plate4506 so as to direct the LO beams back into a PBS cube 4510 and torespective output ports. In this example, effects such as path lengthchanges due to corner cube displacement or thermal changes do notmodulate the LO beams. As a result, such modulations are more likely tocontribute to range errors than systems in which an LO path more closelyapproximates a measurement beam/return beam path within the laser radaroptical system.

Representative Measurement System Implementations

The examples above can be included or implemented in a variety ofcomplete systems. FIGS. 52-54 illustrate a few representative systems.With reference to FIG. 52, a laser radar system 5200 includes ameasurement lasers 5202, 5204 coupled to a fiber-based optical module5206. A pointing laser 5205 is also coupled to the fiber-based opticalmodule 5206 to permit user viewing of a target location that is underinterrogation. A modulator 5208 is coupled to the measurement lasers5202, 5204 and is typically configured to provide a linear opticalfrequency modulation of associated measurement beams. In other examples,an amplitude modulation or phase modulation can be applied, andnonlinear modulations can be used. The optical module 5206 includes areference length such as disclosed above, and a reference optical signalfrom the reference length is directed to a reference detector 5222 thatis coupled to a signal processor 5226 for calibration of the modulationsof the first measurement laser 5202 and the second measurement laser5204, respectively. In an alternate configuration two referencedetectors can be used, one for each laser.

An integrated optical assembly (IOA) 5230 is configured to receive themeasurement beams from the measurement lasers 5202, 5204 via an opticalfiber 5232. The IOA 5230 is rotatable on elevation shaft 5236 with anelevation motor/bearing/encoder assembly 5240. The elevation shaft 5236is secured to a mount 5242 that is in turn secured to a shaft that 5245is rotatable with an azimuth motor/bearing/encoder assembly 5244 about afixed base 5241. The IOA 5230 is also configured to receive probe beamportions from a target, and combine the received probe beam portionswith LO beams corresponding to each of the measurement beams. Thecombined beams are directed to one or more photodetectors, and aninterference (heterodyne) electrical signal is coupled to the signalprocessor 5226 with an RF cable 5227. The IOA 5230 also includesfocusing optics, and provides a common probe beam/LO beam optical systemas disclosed in detail in the examples above.

A camera 5250 is coupled to view a target and rotate with the IOA 5230.A controller 5260 is coupled to the signal processor 5226, the azimuthmotor/bearing/encoder assembly 5244, and the elevationmotor/bearing/encoder assembly 5240. Calibration values, measurementresults, images, computer-executable instructions for rotational controland signal processing, and other data and operating programs can bestored in a memory 5262.

With reference to FIG. 53, a laser radar system 5300 includes first andsecond measurement lasers 5302, 5304 coupled to a fiber-based opticalmodule 5306. A pointing laser 5305 is also coupled to the fiber-basedoptical module 5306 to permit user viewing of a target location that isunder interrogation. A modulator 5308 is coupled to the measurementlasers 5302, 5304 and is typically configured to provide a linearoptical frequency modulation of associated measurement beams. In otherexamples, an amplitude modulation or phase modulation can be applied,and nonlinear modulations can be used. The optical module 5306 includesa reference length such as disclosed above, and a reference opticalsignal from the reference length is directed to reference detectors5322, 5324 that are coupled to a signal processor 5326 for calibrationof the modulations of the first measurement laser 5302 and the secondmeasurement laser 5304, respectively. In an alternate configuration, asingle reference detector can be used for both lasers.

An integrated optical assembly (IOA) 5330 is configured to receive themeasurement beams from the measurement lasers 5302, 5304 to provide afocused probe beam through a cold mirror 5332 to a scan mirror 5334 thatis rotatable on elevation shaft 5336 with an elevationmotor/bearing/encoder assembly 5340. The elevation shaft 5336 is securedto a mount 5342 that is in turn secured to a shaft that 5345 isrotatable with an azimuth motor/bearing/encoder assembly 5344 about afixed base 5341. The IOA 5330 is also configured to receive probe beamportions from a target, and combine the received probe beam portionswith LO beams corresponding to each of the measurement beams. Thecombined beams are directed to one or more photodetectors, and aninterference (heterodyne) electrical signal is coupled to the signalprocessor 5326 via an RF cable 5327. Alternatively, the combined beamscan be directed via an optical fiber back to the laser oven 5306 andthen to one or more photodetectors. The IOA 5330 also includes focusingoptics, and provides a common probe beam/LO beam optical system asdisclosed in detail in the examples above.

A bore sight camera (BSC) 5350 is coupled to view a target along a probebeam axis 5301 with the cold mirror 5332, and a wide field camera (WFC)5350 is secured to the mount 5342 so as to rotate about an azimuthalaxis corresponding to an axis of the shaft 5345. A controller 5360 iscoupled to the signal processor 5326, the azimuth motor/bearing/encoderassembly 5344, and the elevation motor/bearing/encoder assembly 5340.Calibration values, measurement results, images, computer-executableinstructions for rotational control and signal processing, and otherdata and operating programs can be stored in a memory 5362.

With reference to FIG. 54, a laser radar system 5400 includes first andsecond measurement lasers 5402, 5404 coupled to a fiber couplers orcirculators 5405, 5406 to provide a combined beam in a fiber 5407. Abeam from a pointing laser 5409 is combined with the measurement beam topermit user viewing of a target location that is under interrogation. Amodulator 5411 is coupled to the measurement lasers 5402, 5404 and istypically configured to provide a linear optical frequency modulation ofassociated measurement beams. In other examples, an amplitude modulationor phase modulation can be applied, and nonlinear modulations can beused. The circulator 5406 couples the combined measurement beams to areference length 5412. A reference detector 5413 receives the beams fromthe reference length 5412 and couples a corresponding electrical signalto a signal processor 5422 via an RF cable 5423.

An integrated optical assembly (IOA) 5430 is configured to receive themeasurement beams from the fiber 5407 to provide a focused probe beamthat is directable to a target with a pentamirror 5431 that is rotatableon elevation shaft 5436 using an elevation bearing 5432 and an elevationmotor/bearing/encoder assembly 5435. The IOA 5430 and the elevationmotor/bearing/encoder assembly 5435 are fixed to each other via a bridgesupport 5447. The IOA 5430, the elevation shaft 5436, the bearing 5432,and the elevation motor/bearing/encoder assembly 5435 are secured to ashaft 5445 is rotatable with an azimuth motor/bearing/encoder assembly5444 about a fixed base 5451. The IOA 5430 is also configured to receiveprobe beam portions from a target, and combine the received probe beamportions with LO beams corresponding to each of the measurement beams.The combined beams are directed to a measurement photodetector 5460 viathe fiber 5407, and an interference (heterodyne) electrical signal iscoupled to the signal processor 5422. The IOA 5430 also includesfocusing optics, and provides a common probe beam/LO beam optical systemas disclosed in detail in the examples above. A camera 5470 is coupledto view a target and rotate with the elevation shaft 5436.

Fiber-based systems such as those above in which multiple probe beamsand/or local oscillator beams are combined in a single fiber permitarbitrary component placement. All beams propagate in a common fiber sothat motion, temperature, or other environmental effects on the fiberare common to all beams, and are either removed or removable in adetection system.

Measurement Systems Based on Amplitude Modulated Optical Beams

While swept frequency systems offer numerous advantages, amplitude orphase modulated laser radar and laser tracking systems can be used andincorporate the features and systems disclosed above. Referring to FIG.55, a laser radar or tracking system 5500 includes a laser diode 5502that is coupled to an amplitude modulator 5503 to provide an amplitudemodulated (AM) optical beam to a fiber coupler 5504. The fiber coupler5504 delivers the AM optical beam via an optical fiber 5505 to afocusing optical system 5506 and a scanning mirror 5508 so as to directthe AM optical beam to a target 5510. The scanning mirror 5508 can be apentamirror or other suitable optical element. The optical beam returnedfrom the target 5502 is directed back via the fiber coupler 5504 to anoptical detector 5512. A phase comparator 5514 is coupled to theamplitude modulator 5503 and the optical detector 5512 so as todetermine phase shifts between the amplitude modulation applied to theoptical beam and the AM modulation of the return beam from the target5510. Phase information from the phase detector 5514 is coupled to asignal processor 5516 that estimates a target distance based on thephase information. One or more AM optical beams can be used, andplurality of phase modulations can be applied and detected. In otherexamples, frequency modulations (FM) at electrical frequencies can beapplied instead of or in addition to AM modulation.

Laser Radar Methods and Applications

The examples disclosed above can be used to implement the followingmethods and apparatus. FIG. 56 illustrates a representative method oftracking a tooling ball that is secured to a substrate or target. One ormore tooling balls can be secured to a target to provide referencepoints for coordinate determinations. Tooling balls generally include areflective ball-shaped surface in order to provide ample reflection ofan interrogation beam in a laser-based measurement apparatus such as alaser radar.

As shown in FIG. 56, at 5602 a tooling ball location is identified andrecorded based on returned portions of a scanned interrogation opticalbeam. The optical beam can be scanned in a variety of patterns such ascircles, spirals, w's, or zig-zags so as to track a tooling ball. At5604, the identified location is evaluated to determine a position withrespect to a primary scan. The primary scan is adjusted at 5606 so thatthe tooling ball location is at a preferred location with respect to theprimary scan. Typically, the primary scan is adjusted so that thetooling ball location is approximately centered within a primary scanrange. At 5608, a determination is made regarding additional scanning.

FIG. 57 illustrates a representative manufacturing system 5700 suitablefor producing one or more components of a ship, airplane, or part ofother systems or apparatus, and for evaluating and reprocessing suchmanufactured components. The system 5700 typically includes a shape orprofile measurement system 5705 such as the laser radar 100 discussedabove. The manufacturing system 5700 also includes a design system 5710,a shaping system 5720, a controller 5730, and a repair system 5740. Thecontroller 5730 includes coordinate storage 5731 configured to storemeasured and design coordinates or other characteristics of one or moremanufactured structures as designed and/or measured. The coordinatestorage 5731 is generally a computer readable medium such as hard disk,random access memory, or other memory device. Typically, the designsystem 5710, the shaping system 5720, the shape measurement system 5705,and a repair system 5740 communicate via a communication bus 5715 usinga network protocol.

The design system 5710 is configured to create design informationcorresponding to shape, coordinates, dimensions, or other features of astructure to be manufactured, and to communicate the created designinformation to the shaping system 5720. In addition, the design system5710 can communicate design information to the coordinate storage 5731of the controller 5730 for storage. Design information typicallyincludes information indicating the coordinates of some or all featuresof a structure to be produced.

The shaping system 5720 is configured to produce a structure based onthe design information provided by the design system 5710. The shapingprocesses provided by the shaping system 5720 can include casting,forging, cutting, or other process. The shape measurement system 5705 isconfigured to measure the coordinates of one or more features of themanufactured structure and communicate the information indicatingmeasured coordinates or other information related to structure shape tothe controller 5730.

A manufacture inspector 5732 of the controller 5730 is configured toobtain design information from the coordinate storage 5731, and compareinformation such as coordinates or other shape information received fromthe profile measuring apparatus 100 with design information read outfrom the coordinate storage 5731. The manufacture inspector 5732 isgenerally provided as a processor and a series of computer-executableinstructions that are stored in a tangible computer readable medium suchas random access memory, a flash drive, a hard disk, or other physicaldevices. Based on the comparison of design and actual structure data,the manufacture inspector 5732 can determine whether or not themanufacture structure is shaped in accordance with the designinformation, generally based on one or more design tolerances that canalso be stored in the coordinate storage 5731. In other words, themanufacture inspector 5732 can determine whether or not the manufacturedstructure is defective or nondefective. When the structure is not shapedin accordance with the design information (and is defective), then themanufacture inspector 5732 determines whether or not the structure isrepairable. If repairable, then the manufacture inspector 5732 canidentify defective portions of the manufactured structure, and providesuitable coordinates or other repair data. The manufacture inspector5732 is configured to produce one or more repair instructions or repairdata and forward repair instructions and repair data to the repairsystem 5740. Such repair data can include locations requiring repair,the extent of re-shaping required, or other repair data. The repairsystem 5740 is configured to process defective portions of themanufactured structure based on the repair data.

FIG. 58 is a flowchart showing a representative manufacture method 5800that can incorporate manufacturing systems such as illustrated in FIG.57. At 5802, design information is obtained or created corresponding toa shape of a structure to be manufactured. At 5804, the structure ismanufactured or “shaped” based on the design information. At 5806,coordinates, dimensions, or other features of the manufactured structureare measured with a profile measurement system such as the laser radarsystems described above to obtain shape information corresponding to thestructure as manufactured. At 5808, the manufactured structure isinspected based on a comparison of actual and design dimensions,coordinates, manufacturing tolerance, or other structure parameters. At5810, if the manufactured structure is determined to be nondefective,the manufactured part is accepted and processing ends at 5814. If themanufacture part is determined to be defective at 5810 by, for example,the manufacture inspector 5732 of the controller 5730 as shown in FIG.57, then at 5812 it can be determined whether the manufacture part isrepairable. If repairable, the manufactured part is reprocess orrepaired at 5816, and then measured, inspected, and reevaluated at 5806,5808, 5810, respectively. If the manufactured part is determined to beunrepairable at 5812, the process ends at 5814.

According to the method of FIG. 58, using a profile measurement systemto accurately measure or assess coordinates or other features of amanufactured structure, a manufactured structure can be evaluated todetermine if the structure is defective or nondefective. Further, if amanufactured structure is determined to be defective, a reprocessingprocess can be initiated if the part is deemed to be repairable based ondesign and actual structure dimensions and features. By repeating themeasurement, inspection, and evaluation processes, defective parts canbe reprocessed, and parts that are defective but that are not repairablecan be discarded. The particular systems and methods of FIGS. 57-58 areexemplary only, and other arrangements can be used.

In the above embodiment, the structure manufacturing system 5800 caninclude a profile measuring system such as the laser radars shown above,the design system 5710, the shaping system 5720, the controller 5730that is configured to determine whether or not a part is acceptable(inspection apparatus), and the repair system 5740. However, othersystems and methods can be used and examples of FIGS. 57 and 58 areprovided for convenient illustration.

In view of the many possible embodiments to which the principles of thedisclosed invention may be applied, it should be recognized that theillustrated embodiments are only preferred examples of the invention andshould not be taken as limiting the scope of the invention. Rather, thescope of the invention is defined by the following claims. We thereforeclaim as our invention all that comes within the scope and spirit ofthese claims.

We claim:
 1. A beam pointing system, comprising: a first rotationalstage configured to provide a rotation about a first axis; a secondrotational stage coupled to the first rotational stage, and configuredto provide a rotation about a second axis that is not parallel to thefirst axis; a rotatable optical element coupled to the second rotationalstage; an optical system situated to provide a probe beam to therotatable optical element; and a stationary optical system configured tobe stationary with respect to rotations of the first rotational stageand the second rotational stage, wherein the stationary optical systemincludes a laser source configured to produce the probe beam and deliverthe probe beam to the optical system situated to provide the probe beamto the rotatable optical element.
 2. The beam pointing system of claim1, wherein the first axis is an azimuthal axis and the second axis is anelevational axis, or the first axis is an elevational axis and thesecond axis is an azimuthal axis.
 3. The beam pointing system of claim1, wherein the rotatable optical element is situated to receive theprobe beam from the optical system along a propagation axis parallel tothe second axis.
 4. The beam pointing system of claim 3, wherein therotatable optical element has a planar reflective surface situated toreceive the probe beam so that the probe beam is directed to a targetlocation based on a first rotation angle and a second rotation angleassociated with the first rotational stage and the second rotationalstage, respectively.
 5. The beam pointing system of claim 4, wherein therotatable optical element is a pentaprism.
 6. The beam pointing surfaceof claim 4, wherein the rotatable optical element is an air pentaprism.7. The beam pointing system of claim 3, wherein the rotatable opticalelement is situated to receive the probe beam from the optical systemalong an axis parallel to the first axis.
 8. The beam pointing system ofclaim 7, wherein the rotatable optical element has a planar reflectivesurface situated to receive the probe beam so that the probe beam isdirected to a target location based on a first rotation angle and asecond rotation angle associated with the first rotational stage and thesecond rotational stage, respectively.
 9. The beam pointing system ofclaim 7, wherein the optical system is configured so as to be stationarywith respect to rotations of the first and second rotational stages. 10.The beam pointing system of claim 9, wherein the optical system includesa photodetector configured to receive a portion of the probe beamreturned from a target.
 11. The beam pointing system of claim 10,further comprising a camera secured so as to be rotatable about thefirst axis so as to image a target field of view.
 12. The beam pointingsystem of claim 10, further comprising a beam splitter and a camera,wherein the beam splitter is situated so that the camera and the probebeam are associated with a common axis.
 13. The beam pointing system ofclaim 3, wherein the rotatable optical element optical is situated toreceive the probe beam from the optical system along an axis parallel tothe second axis.
 14. The beam pointing system of claim 13, wherein theoptical system situated to provide the probe beam to the rotatableoptical element is coupled to the first rotational stage.
 15. The beampointing system of claim 14, further comprising an optical fiber coupledto the optical system so as to deliver a measurement beam to the opticalsystem, and the optical system is configured to produce a probe beam anda reference beam based on the measurement beam.
 16. The beam pointingsystem of claim 14, wherein the first rotational stage and the secondrotational stage include respective encoders, and further comprising asignal processor coupled to the encoders and configured to determine apointing direction of the probe beam based on encoder signals.
 17. Thebeam pointing system of claim 16, wherein the optical system includes atleast one optical element that is translatable to adjust a focusdistance of the probe beam.
 18. The beam pointing system of claim 17,wherein the optical system includes a corner cube and an objective lens,wherein the translatable optical element is a corner cube situated so asto vary a propagation distance associated with the objective lens. 19.The beam pointing system of claim 18, wherein the optical system isconfigured to produce the reference beam based on a portion of themeasurement beam directed to the corner cube.
 20. The beam pointingsystem of claim 19, wherein the optical system is configured to producethe reference beam based on a portion of the measurement beam directedto translatable optical element configured to adjust the focus distanceof the probe beam.
 21. The beam pointing system of claim 20, wherein theoptical system is configured to couple a portion of the probe beamreturned from a target and the reference beam into an optical fiber. 22.The beam pointing system of claim 21, further comprising a cameracoupled so as to be rotatable about the second axis, and configured toimage at least a portion of a target.
 23. The beam pointing system ofclaim 22, wherein the measurement beam includes beams associated with aplurality of laser sources.
 24. The beam pointing system of claim 21,wherein the first axis is an azimuthal axis and the second axis is anelevational axis.
 25. The beam pointing system of claim 11, wherein therotatable optical element is a pentaprism.
 26. The beam pointing systemof claim 11, wherein the rotatable optical element is an air pentaprism.27. The beam pointing system of claim 11, wherein the rotatable opticalelement is a plane reflector.
 28. A method for manufacturing astructure, comprising: producing the structure based on designinformation; obtaining shape information of the structure by scanningthe structure with a probe beam using the beam pointing apparatus ofclaim 1; and comparing the obtained shape information with the designinformation.
 29. The method for manufacturing the structure according toclaim 28, further comprising reprocessing the structure based on thecomparison.
 30. The method for manufacturing the structure according toclaim 29, wherein reprocessing the structure includes reproducing thestructure.
 31. The beam pointing system of claim 1, wherein the opticalsystem situated to provide the probe beam to the rotatable opticalelement is coupled to the first rotational stage and includes at leastone optical element that focuses the probe beam.
 32. The beam pointingsystem of claim 31, wherein the optical system situated to provide theprobe beam to the rotatable optical element includes at least oneoptical element that is movable to adjust a focus distance of the probebeam.
 33. The beam pointing system of claim 32, wherein the at least onemoveable optical element of the optical system situated to provide theprobe beam to the rotatable optical element is a translatable cornercube, and the optical system situated to provide the probe beam to therotatable optical element further comprises an objective lens, whereinthe translatable corner cube is situated to vary a propagation distanceassociated with the objective lens.
 34. The beam pointing system ofclaim 33, further comprising an optical fiber coupled to the stationaryoptical system so as to direct the probe beam to the optical systemsituated to provide the probe beam to the rotatable optical element. 35.The beam pointing system of claim 34, wherein the stationary opticalsystem includes a photodetector situated to receive a portion of theprobe beam returned from the target.
 36. A beam pointing system,comprising: a first rotational stage configured to provide a rotationabout a first axis; a second rotational stage coupled to the firstrotational stage, and configured to provide a rotation about a secondaxis that is not parallel to the first axis; a rotatable optical elementcoupled to the second rotational stage, wherein the rotatable opticalelement is situated to receive the probe beam from the optical systemalong a propagation axis parallel to the second axis; an optical systemconfigured so as to be stationary with respect to rotations of the firstand second rotational stages and situated to provide a probe beam to therotatable optical element, the optical system including a photodetectorconfigured to receive a portion of the probe beam returned from atarget, wherein the rotatable optical element is situated to receive theprobe beam from the optical system along an axis parallel to the firstaxis; and a beam splitter and a camera, wherein the beam splitter issituated so that the camera and the probe beam are associated with acommon axis.
 37. A beam pointing system, comprising: a first rotationalstage configured to provide a rotation about a first axis; a secondrotational stage coupled to the first rotational stage, and configuredto provide a rotation about a second axis that is not parallel to thefirst axis, wherein the first rotational stage and the second rotationalstage include respective encoders; a rotatable optical element coupledto the second rotational stage, wherein the rotatable optical element issituated to receive the probe beam from the optical system along apropagation axis parallel to the second axis; an optical systemconfigured so as to be stationary with respect to rotations of the firstand second rotational stages and situated to provide a probe beam to therotatable optical element, the optical system including a photodetectorconfigured to receive a portion of the probe beam returned from atarget, wherein the rotatable optical element is situated to receive theprobe beam from the optical system along an axis parallel to the firstaxis, the optical system further including a corner cube that istranslatable to adjust a focus distance of the probe beam and anobjective lens, wherein the corner cube is situated so as to vary apropagation distance associated with the objective lens; a camerasecured so as to be rotatable about the first axis so as to image atarget field of view; and a signal processor coupled to the encoders andconfigured to determine a pointing direction of the probe beam based onencoder signals.
 38. The beam pointing system of claim 37, wherein theoptical system is configured to produce the reference beam based on aportion of the measurement beam directed to the corner cube.
 39. Thebeam pointing system of claim 38, wherein the optical system isconfigured to produce the reference beam based on a portion of themeasurement beam directed to translatable optical element configured toadjust the focus distance of the probe beam.
 40. The beam pointingsystem of claim 39, wherein the optical system is configured to couple aportion of the probe beam returned from a target and the reference beaminto an optical fiber.
 41. The beam pointing system of claim 40, furthercomprising a camera coupled so as to be rotatable about the second axis,and configured to image at least a portion of the target.
 42. The beampointing system of claim 41, wherein the measurement beam includes beamsassociated with a plurality of laser sources.
 43. The beam pointingsystem of claim 40, wherein the first axis is an azimuthal axis and thesecond axis is an elevational axis.